I myself have never made a mod (the only editing I've ever done was making all factions playable in vanilla RTW ), so excuse me for being really dumb if I am... but why can't there be more factions in a modded version of RTW than there were in the vanilla version? If you can have more, I think you could make a much better mod no matter where you start (I'm posting this while downloading AqD's mod ).
If you can have more factions... perhaps Berbers... or Saka? Or a better version of Sarmatians than RTR Platinum? Or even Indians and a map going further East to China?!
Of course, it seems that none of this is possible, so if it was a waste of time. Hopefully it's of some use...