Honestly, I think the mechanic penalties for capital moves are already pretty significant. Production/control losses, if you have a small realm it doesn't matter much and if you have a large realm then a big chunk is going to get hit with reduced stats from distance from capital.
Obviously not enough if the temptation is big enough for us to need a rule for it and for some to have weaseled their way into doing it unpunished.
Realms who move their capital to their border when they used to have a more central capital should not be able to get away with it unharmed.
Distance from the capital only currently affects rather large realms, even if the capital isn't central, as being small will offer bonuses that compensate. I mean, look at Fheuv'n. Iato's off in the corner, and all of our regions are running crazy tax rates anyways without having any distance from the capital problems. Just goes to show what other realms could get away with without suffering any penalties for ditching their capital on their border.
