I think a post Marian reform time period would be a great idea. It would be a very challenging time period as well. However, in order to make it work, I think the game has to have a lot more historical accuracy.

For example, in MTW, the Mongolians would always pop out. There was no way to eliminate them.

In RTW, no such thing happens, even though Rome was plagued by enormous armies popping out from other states! Well, enough of that, I say. Have enormous armies of Mithridates, Italian rebels, Spanish armies under the command of Quintus Sertorius and Germanic hordes pop up in the right year. Have a faction reemergence if the player has conquered these factions out of existence.

That way, we would be able to accurately represent the challenge the Senate had faced - fielding an army with very few funds, rebels popping out all over the place and the enormous armies of the Eastern barbarians.

The game is flawed, currently, because it doesn't show how the most chaotic era of Rome's history really took place AFTER the Marian reforms.