Jevondair: Nice idea, it would also be demotivating to destroy or nearly destroy realms and become large easily.
It would get harder and harder to hold and take these last regions.
Chenier:
I agree that small realms like that can't do much but grow.
Thats why they shouldnt be to much obstructed in it.
This doens't help to make the small realms grow larger nor for them to die.
It creates status quo where no one can do anything but wait or leave the realm.
I'd rather see small realms be conquered or destroyed by other realms then for them to slowly wither away of inactivity.
ideally they should get a chance to grow into hopefully something refreshing.
New realms tend to cause new conflicts.
I, too, would rather see them absorbed by a neighbor than be artificially maintained.
Size in terms of regions is not the main vulnerability, though, it's size in terms of nobles. Small realms used to accomplish things. The Ceded City Alliance was a bunch of tiny realms. But these minuscule realms had as many nobles as many "large" realms do nowadays...
And I think any measure to make small realms more viable should keep with this. Noble count should be the most important factor in a realm's strength, imo. Perhaps the CS/soldier should increase for small units even more than it does not? Based on the captain, perhaps? (don't want to buff militia, though, if anything it ought to be nerfed)
