I thought in any case we would change the calendar, but I didn't think you meant a late start, but sort of a brand new game. I figured if we added different civilizations (which would be a lot on it's own), I figured we would start from scratch... well with the same core. Then agian, if adding civilizations was easy/quick, I think Rhye/ somebody would have included Poland, Ukraine, Israel, Korea, etc already. (But I see how a late start is much more realistic).
With the RF Colonization part, I thought that would just be the first half/third, with the rest being world wars, starting similar to the Napoleonic Wars, early (If we follow history, I see 4 major WWs: 7 Years, the NWs, I & II).
With only a 500 year period apposed to 5000 or 1400, the BtS units and techs would be a little bland, right (I am honestly just guessing). I'm not proposing a RoM game, but like a WolfRev game, adding needed units (coupled with techs and buildings to make them possible), and a
few new features.
-During the Renaissance/ Colonization era, we could use a few RFCE units, (I'm thinking of the Bombards, ships, and a couple of new calvary units.)
-During the Industrial Revolution/ World Wars era, we can borrow some WolfShanze ships (like steamers), light tanks, biplanes, light carriers (I never liked how the only carriers the game had are Nimitz... can you imagine a IJN Nimitz?!??) and some WWI: Blood and Iron units (that Austrian tank is hawt!1!1!!... lol... but seriously.

)
-Modern is pretty good.
I don't think we have to go this far with buildings, however.