so here is the page where you can suggest engines for a empires clone or similar game. I want to suggest polyvox, it's a free C++ voxel engine with support to smooth the terrain so it doesn't look like minecraft. it allows dynamic terrain destruction but has an extremely limited physics engine that is still being tweaked. an engine like this would allow vehicles to take damage that would be visible, also the player models could be rigged to turn into hamburger. this engine would also unlock a cover function much better than walls, such as trenches, fox holes, and dugouts. http://www.volumesoffun.com/polyvox-about/ anyways, please post upcoming/commercial/open-source engines that you believe would be a good fit for a game containing the elements that empires does.
From what I can tell Factions is going to more of a spiritual successor to empires, but not the same game with the same style. I actually was thinking about that after hearing Etho say he wished there was a community driven successor to Empires to another engine, but being one the people who tried to do the same thing in Unity3d but really all I wanted to do was add more vehicle classes and bigger maps. It would be great to see a a community driven successor but I don't see that ever happening, you can never get people to agree on anything here. Edit: That would be pretty nice to have a fully destructible randomly generated terrain.
The engine that gets used is up to the person who does the coding. Anyone who's in that position, knows more than what some random people can submit, and he knows what's going to be the best engine. Hence Jephir has decided on UDK.