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If I had to guess it would be that such a mod would be a huuuuge undertaking which could likely not be done well w/out the ability to mod the game engine. Perhaps some modder will correct me though.
 
I disagree it would be 'more' interesting, this game is Romacentric and about the rise of Rome into Empirehood after all.

But it would make for a fun mod. Main reason i think is that a lot of work is needed and most modders are dividing their attention. Several map redoes out there, small edits etc. Imperium is the biggest attempt at a mass mod and it's upcoming new version will actually go back in time not forth to the decline or division.

So... work + need for a dedicated group of modders.
 
Main reason would probably be the year limit of the game, which is hardcoded. Of course, this could be fixed by just converting AUC into AD and changing the numbers around a bit.
 
I think with a good map (the known world map specifically) a good mod with 3 start ups set could be made (present start, death of Hadrian and onset of the collapse start, and the final split start around 400ad and the time of honorius).

But it surley would need a good mod team, which this game dosn't seem to have :(, and it would also benefit from a CK style region/province set up which must be unmoddable (I presume).

The only way to do it would be to get everyone making mods for the game organized, and anyone interested in the period or with a bit of time to chip in with event ideas, game set up etc. All of which seems unlikley I am afraid :(