You have 20 faction slots available. Make your mod with 19 factions, leave the 20th faction slot empty. Then, make mini-mods which bring a new faction above the 20th slot. Ussually with the AoR units of an area , this is the way I see it. Easy to say, a bit harder to do it beautifully, everybody tells he'll make mods with more then 20 factions, nobody did it until now.

Farewell,
Arbaces.