Well, it depends on mechanics. Currently, there is a system in place that scales FATEs based on previous player participation. Future raids and other content could use an advanced version of this that scales enemy stats, damages, and mechanics based on the number of people in the raid.

For example: LotA: Raid-state: All mobs HP and damage are altered based on total raid participants. Atomos/Phlegethon Mechanics: adjusts the number of people required on a pad based on total raid participants. 2nd boss would adjust the number of people debuffed each round. Basically, remove all effects that target based on alliance team membership, and set up different logic functions, like the equivalent of a party composition.

The current raid mechanics are heavily tied to the alliance system, and moving away from that with alternate logic checks would make it much more feasible.