Hold down shift and then press number keys to access the other factions too.

It'll be a long and arduous campaign to successfully destroy your own super-power empire though, at least I imagine so. Unless it falls into a civil war early on and another faction re-emerges to nick more of their lands.

Although it goes against your realism suggestion, you could give a gift of a million florins to a pauper faction using the .deadringer. cheat and the -ian switch. It'll go crazy bribing armies and recruiting new units, so you'll have a fair few battles with them, probably many against your own turncoat armies. Can get boring and silly though. Still funny at times when you see the AI suddenly bribe half the map under their control, then go bankrupt with civil wars erupting everywhere.