Eh, details. That sort of thing can easily be adjusted to achieve the desired effect, once the basic mechanic is in place.
As to the rest...I dunno, I'd like to see colony TOs return, but I think they might need some reworking to make a bit more sense. And if you're suggesting that we shouldn't allow secessions when the realm is below a certain nobles:regions ratio...well, I might well be persuadable that that should be a thing. 
Takeover and secede honestly seems like the best way to simulate the old colony takeover. Secessions create a ton of penalties across the realm, so they aren't free. Gives the invading realm the time it needs to organize the colony, too.
As for a ratio, I don't really think it's *needed*, and noble count alone can't really guarantee how many of those people are capable leaders. A noble:realm ratio could be used to restrict the creation of new realms, but it kinda seems pointless and seems to me like it could have unforseen consequences. If we want to mechanically disfavor small empty realms, imo we'd be better off focusing on granting realms the means to field a mobile army more comparable to what they used to have back in the days with the old tax system.
