I'd like to suggest one feature: the possibility to swap polygons with adiacent ones.

Let's say I generated a map i'm 99% satisfied with, but i want to move the castle maybe form the center of the city to the outer walls or the coast.

 I know i can use Warp>Displace but that will create geometric aberrations and overlaps, plus it will leave empty spaces behind it or just generally mess up the map. 

Spamming generation would'nt help either, cause since it has infite possibilities it would take me too much time to get the desired result.

I hope it's doable!

I get the problem, but...

Short answer: it is not doable.

A bit longer version: individual map cells ("polygons") are not independent, they affect their neighbours in multiple ways depending on their content, so it's not possible just to replace one cell with another. In previous versions, something like what you propose could be implemented (here is a similar idea), but not anymore.

Maybe in the future I'll find a way to make local changes, though.