I have thought about this for the past few weeks and it would be possible to a limited extent. This is my idea:
You wouldn't be able to have seperate campaigns (i.e. HTW, METW, NTW) due to the limit for unit graphics and texture/backdrop graphics, and sfx.
BUT, you could have a faction for each era. For example, Bronze Age faction (with Chariots, etc.) Hellenic age faction, (with Hoplites, Peltasts,) Roman age (with Principes, Gauls) etc.
I would think for each faction you could have one general unit, one cavalry unit, two infantry units, one archer unit, and one artillery unit. I could plan today how this would work, so you could fit in everything from the Bronze Age to the late Byzantines to Feudal Japan to Napoleon.
You would also be able to mod the music for each individual faction and, of course, shields/flags.
I don't think this would work for a campaign, but I can't imagine how fun a mulitplayer battle would be (your ancient chariots and infantry charging up against 18th century british line infantry, while getting flanked by Mongol warriors and Uruk hai!)
Justin