Lets start and analyze every civ and they unique stuff which “had” to provide unique gameplay.
Delhi - In my opinion Delhi is a little bit more unique in terms of gameplay. Elephants give different feel of gameplay and this is the only civ that usually use healers ,which is sad tbh. Scholars if they got fixed have good
and flexible gameplay mechanics such as healing, taking sacred site, boost atk speed. The melee elephants could be more scary, because his name is war elephants and I think some splash dmg or stomp abilities can be added but for now they are okayish.
Mongols - they have to be raiding civ with unique mangudai unit and nomad gameplay. Nomad gameplay there, but raiding… All i see is spam knights in every match with each civ so they just produce more at the same time. Basicaly interesting base building and stuff but same use of units , exept khan. Dunno what unique teachs thay have tho.
HRE - ïnfantry", "Religion’ civilization. Everyone almost every game spam knights with HRE , even Delhi infantry is more interesting because of the spring ability. You can make surround, hit and run etc. With HRE … boring as hell. Other part is priest inspire abilities. PRIEST WITH AURAS COULD BE HUGE AND SICK gameplay, but this is just mocking. Not worth, the time, not worth the apm, no worth investing in priests at all, better do more knights … i tried it couple of times, no matter lose or win i wanted to try it just to see how good it is and isnt worth at all. The other part is put relic in building so they can shoot stronger and be more hard to kill. I got 5 relics and put it on one secret side to see when my opponent attack what this turrets will do. I wanted to avoid reignitz so i decide to make turrent with offensive purpose. The dmg is stupid low. If this has to be unique, put some numbers in it because 5 towers with relics doesnt do sht to anything and die in seconds.
Rus - I like them. Interesting early mechanic with the wild hunt and overall interesting civ. Monks give flexibilities to the armies, second age landmark give variaty of options. Horse archers is good for raiding, melee knigts are good. Their streltsy are amazing and have SPRINT something that infantry base civ like HRE doesnt have 
. Imagine if sniper units have more variety than whole infantry civ.
Abbasid - interesting stuff, when cammels got buffed and so on they will be good. Can raid with camels, can make sieges without upgrade which is amazing. You can go pure cavalry, camels with infantry for armor aura. Healing castles… overall good stuff.
The only thing that I wanted to play with and to main this civ for is FAITH. I dont like repetative playstyle like mass knights go siege and go archers behind. This FAITH tech provides me that since I love high apm strats.
The bad thing is as someone mention it that is age 4 upgrade. Tried it couple times, if you get attack with 50 horses and sigie behind, even if you have the best micro in the world to hit 20 conversions , all of the priest will die and you will get stomped. Just not worth it , not to mentions that in huge fight you control other units so you can actually bring 5-9 not more. I suggest to be able in age 2 if you take the path for it. Investing i priest is a huge deal so wouldnt be op and they can really get sniped.
China - Interesting sim city overall. Unique units are stupid. Zhu - ko - nu doesnt feels soemthing special. Fire lancer as they said should be raiding unit so instead of making in just hit once and go back, just make their basic attack deal splash dmg and bonus to villagers, something like that. I dont see any purpose of it now instead of gimmicky playstyle. Also if you want to make some civ siege oriented give them interesting siege playstyle, dont just buff the same units like now. Now if you reach this time I think isnt fun for both of the players. So if want to be siege civ, make different units instead of casuals. The last unit, grenadier… i never saw it on a game so idk about it.
France and English i think even if they are simple they are okay. You want strong , beefy good locking knights go for france, you want long range archers go for English.
The bad thing for me is Knights and Horseman should be two separate units. Knights should be slow moving, hard hiting, heaby unit for fight, horseman should be fast, raiding, concave oriented hit and run unit. When I see sometimes knights that can run as the same speed like horseman i am laughting so hard. Why would you even make horseman ? They can even make horseman attack while moving for better raiding.
Another thing that is dull in my opinion.
Knights and lancers are the same unit with different name. In reallity they are two separate units so make them different this will stop boring knight vs knight matches.
Another thing wich can be changed is the boar. No one ever touch the boar, yeah you can go greedy and take it but if you get cought you are -10 villagers behind. So make the pro scouts take parts of the boars so you can bring it. So lets say if you have 6 remaining scouts from the deer hunt, go and take part of the boars. This will put something people will fight for and will boost your eco alot.
Thats my two cents. No hate here, just want the game to be better and civs play to be more unique which now isnt at all.
Best wishes to the devs and everyone in the forum