Well, you could use cheats to make AI factions expand more historically. Give money to the faction you want to expand, use diplomacy to give and take settlements from faction to other faction and relocate their armies with move_character cheat (I think it works in RTW?).
That way you can have Parthians defeating Seleucids and expanding to west, Macedonians and Greeks staying in Greece. Romans expanding to west, south and east... etc. But it requires lots of extra work and can make campaign boring.