Ideas for realm expansion (unordered):
- Timegate on building cairns
- Increased upkeep on cairns
- Localised resource maintenance for cairns
- Enclaves are destroyed
- Enclaves are ignored
- Enforced dimplomacy
- Soak/hp of cairns are too high, the soak raises too fast.
- The cost of last minute diplomacy should increase with stacks.
- Cost of cairns is too low.
- Naked alts with stones shouldn't be able to repair cairns, it should take at least local resources or a skill.
Timegate: Quite simply a realm can't build a cairn until x amount of time since the last one was built. This is IMO a crude fix, it is (I would assume) easy to implement and adjust however.
Increased upkeep: The easiest solution to implement require no extra coding.
Localised resource maintenance for cairns: Each cairn requires regular attention, this could be seen as a tax, in addition to the upkeep requirement which is automatic across the realm. A cairn would require visiting and some form of tax paid to it, probably in the form of experience, if the local population are happy with the realm then they would be willing to maintain the local cairn, otherwise the government would have to visit and either extract the tax or pay it themselves. Of course this would require much more dev attention and would then need balancing, but provides an increased incentive for interaction between government and the plebs.
Enclaves are destroyed: Any cairn in a portion of a realm that isn't contiguous with the coronation stone quickly decays. This encourages realms to have more than 1 cairn adjacent to each other meaning snaking borders is risky.
Enclaves are ignored: Less drastic than enclaves are destroyed, any cut off portion of a realm doesn't generator authority or receive bonuses/bonii. It would be relatively easy for a realm to repair this and rejoin the disconnected part, without the urgency of the decay from the other suggestion.
Enforced diplomacy: You shouldn't be able to build cairns in a zone where a border was contested, perhaps even further if you use last minute diplomacy.