A simple raid boss event like echokinetics had back in c4 was more than enjoyable. And everyone got something out of it.
An npc spawns you register (or by voiced command). Then everyone gets teleported, the raid spawn after a couple of minutes (antharas for example) everyone fights it for 60 minutes or a fixed time. If the boss isnt killed before the 60 minutes if its hp is less than 50% you get medium rewards, if its hp is more than 50% you get low rewards and if you kill it you get high rewards. Something like that.
It will be kind of a fresh thing since most events these days are pvp based.