Regarding the idea of limiting takeovers by noble density. A realm needs a minimum size of 3 regions, say a city and 2 rurals to survive. So if you set the density to 2 nobles per region then you need 6 nobles to sustain a minimum sized realm. All the small realms in Dwilight have around 5 or 6 knights so could continue to survive in a small area of 3 regions. If however you set the limit to 3 nobles per region, Morek, Arnor and HD will soon cease to exist or at any rate will remain chronically short of food.
None of this sorts the problem, because even if you do shut down small realms all you will have is larger realms isolated by long distances with vast uninhabited areas in between. So no effective interaction between realms.
