Ensemble Studios already knew they will screw it a lot if they didn't do medieval age. They know what a failure was AoE 3 era, not the gameplay, not the design, The Era. They could start from there a new 3D era for ensemble studios gaming, however making WW1 or WW2 is more difficult since it will be probably a copy from Empire Earth or Command and Conquer.
Honestly the most interesting aspect of RTS Genre are: Fantasy, Roleplaying, Ancient History, Medieval History.
The Gunpowder Era in RTS Genre doesn't sell too much unless you are ready to handle such amount of entities like Cossacks. Gunpowder era is more for First Person Shooter or Spionage.
A WW1/2 game would fit perfectly with the Dawn of War gameplay. But in a game like AoE 2 or a Gameplay like Ensemble Studios does, is just a basic genre RTS. Unless they are going all in with a Settlers Economy management in AoE 4 mixed with the micro managamente of AoE 1/2.