There are huge swathes of empty real estate on Calradia's map. Since Warband's "Custom Settlements," and their adoption by a lot of mods, I'm surprised this isn't a no-brainer to Taleworlds.
We SHOULD be able, with sufficient resources and time, build a brand new faction from scratch. Hell, it would make generational playthrough actually meaningful.
Start with a tiny village-in-the-work that nobody cares about. Once you have enough hands to do the work, throw up some crummy low-effort castle just so you have a lord's hall and lord over the dirty peons in style.
Once the place develops into a genuine honest-to-Harlaus town with some degree of prosperity (you know, profitable from a holder's point of view), maybe only then other factions actually start looking your way for that sweet addition to their kingdom.
It would give the player time to derp around without having to worry about war declarations while earning all those denars necessary to support such endeavor.
Anyway, in whatever shape or form, I really hope player-founded fiefs make it into the game. Though after the official removal of castle building... meh