Caiyun is:
3 regions from Donghaiwei
5 regions from Nifelhold
5 regions from Muspelheim
4 regions from Aegir
5 regions from Freke
5 regions from Mimer
4 regions from Uterstrom
5 regions from Balance's Retreat
6 regions from Flowrestown
All regions, except for HD, have at least 1 non-owned region between themselves and the region.
For Astrum, that's 2 rogue regions to reconnect it, but it also already has 2 core regions that are rogue (Moses End and Libiddo). Morek has Nidhoog's Mark that it might reconsider taking. Arnor has Stratford. Sol, assuming it's displaced... you'd need a minimum of 8 colonists just to have the density required to expand all the way to Caiyun. And then you'd have a linear realm that would likely get wrecked by rogues.
Yes, it is feasible for a number of realms to take over Caiyun. But, in my opinion, the only way to hope to actually keep the region, instead of losing it to rogues again and again all the time, would be from Donghaiwei, with at least 12 nobles.
Where will you get those 12 nobles, though? All of the north-eastern realms are already fairly stretched, maybe Arnor could migrate but I doubt that's the plan. Thus, any expansion towards a realm that will have a fair chance of actually keeping Caiyun will require sacrificing lands somewhere else. Is Morek going to give Muspelheim to Arnor or Avernus? Is Astrum going to give Eidulb and Libidizedd to Westgard?
Again, I can appreciate the RP drive to get that region. But mechanically speaking, you can't have it all. There are monsters, density mechanics, and diplomacy at play. Either this new realm will be a miserable tiny realm like HD, that we really shouldn't be striving to multiply, or it will suck the life out of one or many existing realms.
