Even the Wars of Liberty mod team has said they didnt want to have it added. Its just not good as an official addition. It is what a mod should do, change the game in a way it isnt the old one anymore and add new cool stuff that stands on its own.
You have the mod, you can still play it, no need to add it to DE at all.
Because the USA looks strange without Mexicans. And if there were 2 North American civs, it would be good if there were also at least 2 North American civs. DLC with Mexicans, Brazilians and Gran Colombians (possibly Argentinians too) could be called - The American Revolution.
Mexicans, Brazilians (and Argentineans if they were also in the DLC) could have their own style of architecture - Latin American.
I would like the Battle of the Alamo in Historical Battles (Texans could be USA civ with Texas flag).
A mod is a mod, and let it stay that way.
Rather have the US look strange then the whole game.
Historical era of AOE III was supposed to be 1400s to 1850s (Sepoy Rebellion campaign). Empire of Brazil is Victorian era timeline nation. Adding it would make the era intended more focused on Victorian Era. Although a Wars of Liberty mod fork or clone for DE sounds good.
Go back to play Warchief campaign so you wonât say that again. 
How about cancel Industrial Age to make everything ânormalâ? 
Where did I say to remove US? It was a mistake civ but I dont expect themafter their release to delete them. However they are definatly a mistake to have included both for people now wanting stupid civs before important civs and civs who dont fit in the timeline of which US itself does not fit as they are simply british.
I think you could spice up the Imperial Age in AoE 3. You could make this Age model after the Victorian era - new buildings design, some new generic units etc.
Overall, I think the buildings should look more attractive.
Age 1 - Renaissance
Age 2 - Baroque
Age 3 - Classicist
Age 4 - Romantic, Historicism
Age 5 - Art Nouveau
Alternatively, European civs structures would have a different appearance on non-European maps, and would have a continental appearance on European maps.
Sorry for my mistake,I said cancel Industrial Age.
I shouldnât add <> with âIndustrial Ageâ, it just disappear for no reasonsâŠ
Itâs not impossible to fix nations like USA.
For example you can Stipulate the revolution nations must start with Commerce Age and move the advanced units cards from Fortress Age to Industrial Age.
I absolutely agree European nations should be improved as Nizam Fusilier, and I also insist the Gatling Gun and Native allies cards should be move to their currentâs next age.
Although I support to make revolution nations become playable ,but I always support every ideas to prevent âWEIRDâ. 
I mean they could have updated the revolutionary civs in the sense of adding cards and allow players to make a revolt deck.
As what you are describing with comerce stuff sounds like just the revolutionary system.
I wouldnât mind. from the campaign timelines and cards/civ leaders, aoe3 encompasses roughly 500 years of history, from 1400-1900, from before the fall of Constantinople right up to ww1. the question is if it can be an interesting and unique enough civ to stand out, if it can bring with it a good campaign, and if it makes sense with other potential dlc civs.
On paper I did not mind the US as I thought it would be a fairly cheap civ to make because it would reuse alot of Euro assets like architecture and units. It would be much easier to make a US civ than say Persia which would basically need new assets for everything. So the US would not be âstealingâ the slot of a more deserving civ because the cost to create them would be very different.
In reality though the US got very special treatment with 10+ unique units and unique mechanics. The only thing they held back on was giving them a unique architecture but they still got a unique capitol, saloon as well as the usual unique church.
They could do more post-colonial nations reusing US mechanics like immigration cards but Iâm not sure how well they would all fit. I dont know enough about Mexico or Brazil to know if that would make sense.
Of course if you made me choose âBrazil or Persiaâ or âMexico or Ethiopiaâ Iâm going to pick the latter in both cases because of the greater variety they would bring to the game but I know development is not that simple.
there is a massive difference between 1876 (when the game actually ends) and 1900.
by 1900 dreadnoughts where starting to see the light of day
riffles by now had become repeating riffles, reducing reload down to just a couple of seconds between shoots.
Radio has been invented.
electricity was now a fact of life.
machineguns had been invented.
also while the game technically ends with the battle of little big horn, in truth most nations in the game at best could be described to the 1850s, in some cases earlier.