You can't expand beyond 20 regions in this game. Also there just aren't enough people to expand that much even. There aren't anything special about rogue regions so people don't bother exploring them.
If the devs really want to minimize the size of realms, although it sounds tedious as hell, you can just reduce all non-human owned regions to Storms Keep's level. 100ish gold and 100ish food for all non-human regions. Nobody would bother taking them.
