I was considering this.
A Joint Coalition of Factions united under a common goal, but unlike other coalitions which are held together by simple need. I was thinking of a shared power sort of system. A brief outline of my thought.
Faction A - A Small 1-5 Member Faction expertise in building vaults, and safe bases
Goal of Faction A
Make a Vault Roughly Chunks big for 5 Factions to place valuables in it. Each faction could own a piece of it. Since this would be exclusive to the owners of the faction there would be very little chance of grief because even if a faction leader betrayed there would still be no way for him to blow something up on anothers land. This concept is one that takes quite a bit of logical reasoning and not first impulse. If you a faction of per say 10 was raided by 2 Prot IV Pro Raiders. And lost alot of power and had alot of land you vault would be immediately at risk. With the Joint Vault an Allied factions can guard your things before anyone can get to it, and the chance of anyone finding this vault at 50000 Blocks is unlikely. This is truly a Very unlikely part of my draft as very few factions would be willing to risk it. The next parts are far more likely, and far more beneficial.
Secondary Goal of Allied Coalition is Information sharing
Half the time factions have information other factions need, but hold onto it because they feel a raid is impossible, when most of time its not. As well as in the view of the owner sending 2 Prot IV Fighters from 5 factions to destroy a faction is far more safe than sending your entire faction with axes and shovels to fight. This system has worked before on another Anarchy server, except on that server Gdion drafted the idea. With his plan we were able to wipe out all but two or three Strong factions.
The Third goal of the Coalition is to dominate the economy and the market
If you can stop good wepeans from being sold to rogue factions. You can stop them from becoming strong and they are more likely to become united with the conflation. As well as when the Market opens the coalition will be able to buy any supplies they will need seeing as they'll have the buying power of not one faction, but many.
---All Suggestions welcome. If you would be interested in forming such a Coalition respond as well