The time frame chosen for the game was/is 500 before, 500 after, "0 A.D.", so most of what you suggest is still possible. It was only to try and lower the work-load a bit that it was decided to split the game up into two different parts, not that e.g. Imperial Rome was deemed uninteresting  As for the exact time when something was created I believe there has been some liberty taken with that already, but I would guess that leaving the Colosseum for the next part is a better choice