This is one of the first suggestions I've written in the college computer lab! Fortunately for me, there's no shortage of computers, or I'd have to cede control of the keyboard to someone who actually needed to use the computer for academic purposes.
Anyway, I'm afraid to suggest any new gear because, well, it might be underpowered, what with the way OOO is taking the Chaos set and that ridiculous Black Kat Cowl. (Poor Mercurial Demo Helm, it feels so sad and neglected... it's OK, you'll always be my best friend in GitM, Merky!) So instead, I'll suggest bosses. OOO can do bosses well, though I wish they would do bosses more often.
ASPHYXIGAN
Destructive Despotic Toxin Totem
If you had a place that was already sufficiently covered in poison gas so as to be uninhabitable, what kind of boss would you put there? A giant Grim Totem, because we already have a place like that called Concrete Jungle, and the place is full of those obnoxious Zombie-reviving Grim Totems. Asphyxigan, a massive tower of stone with an angrier-looking green Grim Totem on the top, is smack in the center of a circular rooftop (perhaps a nuclear power plant?), flanked by two Vile Howlitzers, two Hurkats, two poison Bombies, and two Toxigels, all of which respawn. At the beginning of the match, a loud voice (AKA red "shout" speech bubble) will shout some cliche "who-dares-enter-my-chamber" crap, and then the battle will commence.
Asphyxigan's normally invincible, and his attacks consist of blasting you with poison gas, buffing the little jerks that are working with him, firing a spread of poison bullets, and, of course, reviving the Bombies almost constantly. So how do you beat this guy? Simple. You'll notice eight tough-looking buttons around him, most of them pressed down. If a Bombie explodes near one, the switch will be pressed down, and a Rocket Puppy will come out near it. You have to get the Rocket Puppy to hit Asphyxigan a few times, and the stone armor near the bottom will be blown up sufficiently for you to attack the glowing green crystal inside. After a short time, Asphyxigan will cause the stone armor to regenerate. Asphyxigan counts as an Undead for damage purposes.
There are three phases of this type of battle, with each new phase bringing a new twist. The second phase adds an attack where Asphyxigan creates a trail of poisonous thorn roots that chases the Knight with the lowest health, and the third phase changes the Hurkats' AI so they try to destroy the Rocket Puppies when they spawn. (Poisoned Rocket Puppies take more rockets to blow up the armor, so I'd kill off the Hurkats ASAP.)
After the third phase, the top of Asphyxigan will be all that's left. In a ridiculous cutscene, a Toxilargo will fall down and completely engulf Asphyxigan inside it, with Asphyxigan's immediate reaction being exactly what you'd expect. The final phase of the fight is against this Toxilargo. Of course, Asphyxigan will power it up, so it will have a metric crapton of health. The Toxilargo gets new attacks, like creating that annoying Thorn Root chain that was introduced in the second phase, covering the entire arena in poison gas, and doing a super jump that causes massive Piercing/Normal/Stun/Poison to whatever unlucky soul is caught under it. Its Spike Spin gets more spikes and its regular Thorn Root attack goes out further.
Also, during the Toxilargo phase, the minions will change. In place of the Hurkats and Howlitzers will be two poison Deadnaughts, and now four poison Bombies, and four Toxigels. Oh, and it now counts as a Slime.
Once the battle is over, the Toxilargo will explode, hurling Asphyxigan into the air, then causing him to crash-land in the center of the arena and shatter into pieces! Cue rewards room opening, and that little "daa da da daa da da da da dadada da" that plays when you complete a battle room.
Rate and comment. Now it's time for Calculus. See you after then!
I will only truly enjoy a boss like this ONLY, AND ONLY IF it spouts Sinistar quotes mid-battle in shout-text.
I want a giant tower of rocks tell me to RUN, RUN, RUN, RUN, RUN, COWARDS.