I read the comments, and soem critics are fair. I would like to make something clear with this long list of proposals: I don’t mean for all of them to be implemented in one big patch, and they are moreover not all on the same level of importance. Some are I think necessary for balance (e.g. a Portuguese change to the early wood bonus, or the monks, or the Organ gun), some are necessary to prevent unfun or unfair strategies to play with or against (Feitorias, Flemish revolution, etc.), and some are simply to add a bit of flavour and variety to the game.
This being said, here are my replies:
- General:
I agree, although the decrease of the anti-elephant bonus should only apply to halberdiers, and be soft enough (e.g. -3).
Yet they are very rarely seen, and are easily countered by siege and cavalry.
Yes, I would rather have a gold unit rarely seen be a bit more efficient against elephants, and the last upgrade of a trash unit be a tad less. Heavy Scorpions already have 8 PA. 
I know it is an inteded weakness, but it could very much remain for the non-elite, and not be the case for the elite version. Reducing the price of Elite may be interesting, but even doing this, the Steppe Lancer is always competing with other strong cavalry units in Imperial such as the Hussar, Paladin or Keshik. To be clear, I don’t propose to change anything to the Castle Age unit. Elite are simply never seen.
It is not “necessary” indeed, but the stone upgrades, especially shaft mining, are rarely researched. I also just want to start the discussion about why these upgrades are more expensive for a lesser effect (particularly on stone).
I mean, the Feudal Age upgrades don’t need to change really, but if we were being consistent, they would. Double bit axe is 100f, 50w, and Horse collar is 75f, 75w.
- Aztecs
We can discuss the monks, although I am not too sold on that. Strong monks are only a problem in a combination that can not be stopped, like Bohemians (with insane pikes and Houfnice) or Portuguese (with Organs and siege). It is a strength of Aztecs, but the nerfs to the Aztec eco put them in a good spot in my opinion. Buffing Jaguar is nothing “necessary”, but they are rarely seen, and the cost in particular is a problem considering they are created at a Castle and are so much more expensive than Champions.
I fully agree to this.
- Bengalis
Perhaps people are right to criticise 25% food from trade as a team bonus. However, it seems fair the bonus would be on par with the Spanish extra-gold generation. Moreover, when trading, gold is the number one resource wanted. This being said, the Spanish team bonus is probably overtuned itself and could be nerfed too, say to 20%. From a pure Bengalis late team game balance perspective, I see how buffing massively the bonus is not necessary. Perhaps 15% as a start.
Balance-wise in 1vs1 open maps, the problem is the early game. However, I don’t see anything wrong with this permutation you suggest. Perhaps it would make people more hesitant in researching the UT though.
- Bulgarians
Honestly here I am a bit shocked you seem to think Konniks are so good. They are basically more expensive knights, and Bulgarians have better knights anyway. Sure, there is the dismounted konnik, but they are a squishy infantry unit which hardly makes a difference (lowkey does against pikemen).
Bagains is also very late game tech. Perhaps the measures I am proposing are too extreme, but an adjustment to the unit through a dismounted konnik buff seems fair to me. The population change alone may suffice. My logic here is simply that you don’t want too much population to be occupied by a unit of lesser quality than what you could be getting, but also don’t want to execute your own dismounted konniks if they survive a fight. This would only come at play when population efficiency matters.
- Burgundians
Disagreed, I think touching their eco is the best way to nerf them.
I like AOE3, but prefer AOE3 to be doing its things while AOE2 does its own. I don’t having anything against the effect you propose, but still don’t like the instant conversion of villagers into military units. It could also just unlock Flemish Militia and make TCs work 25% faster. Unsure of the balance of that, but in late Imp I suppose it should be fine.
- Celts
Fair question. It is not great, especially for the Celts, but I don’t quite know what to do with it. An idea could be to make it a civ bonus, and the new Castle Age UT to be siege firing faster. This would nerf the Hoang rush, but unsure if this is a good idea altogther.
- Cumans
All right, so first I agree the Cumans are difficult to balance, and they have favourable matchups in certain settings, mostly due to their Feudal boom. This comes down to what I proposing with this list again: here, I focused on Elite Steppe Lancer play with Cumans. I think once everyone is fully boomed, Cumans are far from being insane. The balance problem you mention are all fair, but are almost a different problem (which I did not address here).
We can always talk about solutions to that: a “mini-TC”, with less HP, and which would not produce Villagers as fast, at least in Feudal Age, or even ever. Or increase the build time of the Feudal TC even more?
As for Elite Steppe Lancers, we can talk about numbers. If they get more armour too, then I agree two additional boni here is way too many. Otherwise, either the Elite upgrade could be free, or perhaps a possible cost reduction should not be as strong as 25%. But given their current state, and this being locked behing a UT, 25% cheaper seems fine to me. The Berbers’ cavalry if 20% cheaper, without any UT, and these are three commonly seen and used units, contrarily to ESL.
- Dravidians
I must say I am not exactly sure of the course of action to take for them, and I agree they a bit of “something” (probably called cavalry to be honest, but this is a defining trait). Your solutions are both interesting. Even cheaper barracks techs would let them double down on infantry, which is still difficult to justify in most cases.
I think their problem is a 1vs 1 speed one, hardly solved by improving their elephants. Better eco may also do, but given how strong they already are on water maps, I would not dare touching that.
- Ethiopians
Integrating Royals Heirs into the Elite upgrade is a good idea in my opinion, but I did not mention it as I don’t know how to replace it. I agree the civilisation is decent, and my proposed team bonus changed is far from necessary, but more “flavourful”, as it would help getting a better map exploration, and secure possible side resources for the late game.
- Franks
Yeah, again, this is just a “fun” idea thrown in there, but I don’t think it makes sense (as mentioned in the orginal post). Yet, I would not give them Arbalesters, for balance reasons.
For CA, it does not mater balance wise, but it is silly that they would have this bonus. As scaling the bonus, that would be quite harsh on them, and I would rather nerf them with early economy (berries).
- Goths
Do they need to though? Do you mean: for trash wars? Historically, the Goths also had good cavalry.
For Goths, I am not proposing to enforce all these proposed changes together. Making their Villagers stronger would participate in making them a bit more solid and harder to raid. Better defense vs wild animals seems fine too, but only applies to early game.
I suppose it could be capped to just boars if need be (and then would be 170f). For reference, the Mayans get +15% on everything (so also sheep, just considering food), and also get their additional Villager from start. Even the Tatars get +400f from sheep. Honestly, if it were just this bonus, I think it would be fine. 25% is not that huge of a bonus.
More generally, the problem with Goths is to have only one type of unit to excel with, and such polarising matchups. This is why I think giving them Plate barding armour would be interesting. Their “gap”, during which they fall off, extends from mid Feudal Age to mid-Imperial. This is probably the longest of any civilisation. I also thought of taking cheaper Blacksmiths from Bohemians to give them, for a bit of a breathe in Feudal, but not good enough.
- Incas
Yeah maybe, but I am just concerned for the Eagle Warrior bonus to be too good.
- Italians
Perhaps, what do you think?
Honestly, I don’t think they are too weak anymore, especially given the cost and training time. Perhaps my propositions for differentiating the non-Elite from Elite was too harsh, but right now they are almost the same.
- Japanese
I agree it is not historical, but it is defining for the civ. However, it is too expensive in my opinion, and rarely seen. Why would 1s be OP? They really lack a late game strong unit to force engagements, and being able to quickly retreat with strong siege is, in my opinion, a way to force engagements in the disfavour of enemies who could just be faster and take better fights otherwise.
- Khmer
Which FU light cavalry or cavaliers can accomplish perfectly. Also, this partially (but not fully) address the problem of this civ, explained here.
- Koreans
Or reduce the price of the UT.
- Magyars
Okay, that is fair.
- Mayans
Maybe, but this would not come into play before the late game, while the nerf I propose is very much across the board. That being said, I don’t like tuning down this bonus myself, as it is very unique and defining. But they are so very dominant on open maps…
Honestly, Mayans are strong, except if they need Champions (which they still would not get with supplies).
- Persians
Maybe. This is an interesting idea, although returning cost technologies don’t have a great record so far. But I would not mind trying.
I hear the argument about water, but otherwise, yes, the point is to make them a bit faster. Looking at other eco boni out there, I think it is reasonable. I think however Persians need a little bit of an early bonus, given that the meta is always getting a bit faster, and Persians have always been a bit slow. Looking at where they are now, a little push could do. Adjusting starting ressources to compensate this may be necessary.
Kamandaran is fine, why replace it? I guess it could come in Imperial. I suppose you want to give them Bracer for cavalry archers? This would make them extremely versatile, but not really solve their major early game weakness.
- Poles
I think there are various ways to address their strength. They have +2/+2 armour in Castle Age. This is incredibly oppressing given their price, so I would rather see the cost increased and the armour decresed for non-Elite.
- Portugese
This bonus does not really come with any justification for the Portugese specifically… and they can be a slow civilisation on open maps, that’s fine, especially given their gold discount being relevant from Feudal Age. I honestly don’t think their versatility is that much of the issue (especially not so infantry), but the insane Organ/monks/siege pushes on Arena, and the Feitoria.
One way or another, non-Elite Organs should be nerfed in my opinion. Hitting the speed would make them a bit easier to counter with mangonels. Also, it would not make Portugese map dependent, but the Feitoria. It would solve its two main issues: the infinite wood on water maps, and the inifnite gold and stone on all maps (especially closed 1v1).
- Sicilians
Yeah, why not, I suppose your option is a slightly better buff, although the threshold for addressing a tower rush would still be the same. I suppose it is less problematic these days. Unsure what you mean about letting Serjeants build Donjons?
- Spanish
The Missionary is not really.
Moreover, I complete this section here:
- Trade bonus reduced to 20%.
- Faster constructing villagers only applies after the first TC is built. This is of course a Nomad nerf.
- Turks
Janissaries are very powerful, but in open maps I think are fine. I suppose the non-Elite one could take a bit longer to train for Arena.