If you don't add anything at all there is a good chance that you will run out of leaders and civs on large maps.
Its not at all uncommon to have up to three separate revolutions/factions break away from a declining empire over time with RevDCM. Especially on large map/marathon games.
I don't think the number of civs is a problem, since most revolutions in this mod just split an empire into two factions of the same civ type. And all minor leaders are included, so every civ should have at least two leaders.
At all, I have not had too much revolutions in the games I played up to now, but because of my computer I play on Standart and Epic. I don't know if bigger maps run out of leaders.
Stopping barbarian cities from spawning into civilizations will help some here but I doubt it will be enough. Stopping barbarian civ spawning also removes the single most strategically challenging aspect of RevDCM. Having an Orc city spawn into dwarfs would I agree be really dumb, but at the same time it is a shame to completely disable such a challenging and fun game feature.
Should diversify the barbs, like we'd tried in RifE
I would love that. I think it shouldn't be very hard to split barbarians, just give some cities and units a different artstyle and race and tell them just to defend their own cities. How did you solve that in rife?