Disclaimer: This is purely aesthetic, and is mostly for fun and casual gameplay.
Whether you like the addition or not, the US is the most “advanced” civ visually with Civil War-era units and steamships, while most European civs visually stalled at the Napoleonic era (and Japanese stalled at the sengoku jidai, and Aztecs did not even develop).
Gameplay-wise it could be balanced, but to keep the style a little more consistent, there are a few things that can be added just for more flavor:
If some unit looks too advanced compared to the overall design of the civ and cannot be placed anywhere, it can be unlocked by a card or technology, with limited accessibility. Nizam fusilier is a nice design (though very weird), and other church cards can have a similar rework.
Card-specific units: you can keep steamships and ironclads as unique units for the US, but similar units can be sent by cards in the industrial age, etc., with a certain cost like privateers. I always found it weird in the original game why revolution nations look more advanced than those that go imperial.
Visual upgrades: all units in the original game have the same model for guard and imperial levels. In DE, some new units gain the new imperial model. This can be done to most other units to push the timeline to ~1850s.
Some more unique late game technologies.
Ways to avoid significant impact on balancing:
DE devs actually did a good job adding some new features to the imperial age, where most competitive game would not reach. That’s really a nice touch.
More on this:
In AOE3 units typically do not evolve into another unit. So Janissaries remain Janissaries and just get more decorations when advanced in the “Industrial era” where they became obsolete (like all Aztec units). So it’s weird if they suddenly switched to a westernized line infantry like the Nizam Fusilier. A way to circumvent this is to make the latter a different unit.
AoE3 already has a fort prefab you can plop down in the editor, they could easily make a map where players start with that instead of just a normal TC. The prefab is a small fort with a TC and a market inside. They could have variants that use different fort walls for different civs, as appropriate.