How about a Metroid Invasion? There are plenty of metroids to have a variety. Ranging from as little as Miniroids (baby metroids in Prime 3) to regular metroids, hunter metroids, fission metroids, then to omega metroids (they would probably be rare). Theres also Alpha and Gamma, but I dont remember where they fit in the ranks. Heck, maybe a rare drop would be a summon for a Metroid Queen boss.