I personally want to see tortomega back. I miss fighting that big lug as opposed to 50 little guys. Make him a mini boss, Everyone loves mini bosses! Give him a small chance to drop random equipment.
Another idea is to have a treasure box slime enemy who drops rares but is really hard to kill!
Giant mechs with giant drills as weapons which deal piercing damage.
Like most people, I would like to see new enemies that force players to use a wider array of the available equipment. What this means first of all is more piercers! I think they would fit in most neatly in the beast and slime families, both because they need more members (only 2 types of beasts, lack of difficulty w/ current slimes) and because pierce-resistant gear is made from their materials (in the same way shadow-resistant gear is made from ghost bells and reaper ribs, for example).
So, perhaps some giant killer bees on the beast side, or some spiky balls that roll ricocheting around the stage to fill out the slimes.
Also, maybe I'm a masochist, but I think an undead boss could be pretty cool. Maybe a giant skeleton with everyone's favorite phantom pals in the wings, who utilizes a status effect other than friggin' fire. Ice, curse, or maybe even the unused "doom" effect?
Are Vog cubs and Owlite mages. Maybe some water-themed monsters too?
I'd like to see new enemies that aren't just the same guy with a different look and status effect (wolvers, zombies, etc forever). Slimes are my favorite class of monster because each elemental type at least acts different from the others.
The Shadowknight is a dark enemy, composed of flowing shadow-like material, with glowing red eyes. Its shape resembles a Spiral Knight.
The Krabbe is a large crab with a coral on its back. It has a small claw, and a large claw.
There could be families for each of them.
I totally think there could be more monsters.
.Caterpillars
.Dragons
.And Enemy mage mech knights
^.^