They could do something like the events we have now. For one week your guild kills purple/red/blue/etc. troops together to work toward some sort of traceable goal. If completed your guild gets a special bonus. Or for one week your guild works together to take down a fixed enemy team. Complete that so many times in a week and you get a special bonus or emblem or bragging right.
That’s still separate but together in the best way possible, while still promoting unity. It’s probably as close to raiding as we could get in this game.
Each week would have a different story theme. The curse of Queen Mab, The rise and fall of Valkyrie, Imp Madness, etc. Just to give some simple examples.