One thing about DS2's multiplayer that I found disappointing was the lack of Necromorph variants. There were only four Necromorphs in MP, five if you count the AI-controlled Slashers. I think there should've been more, maybe unlockable variants that you get as you level up. Here are just a few I think should've been in multiplayer:
Infector: unlocked at level 5. The Infector is able to…well…infect other corpses scattered throughout the map to create more Slashers. Its attack is the same as in the campaign, hopping on its victim, trying to stab him with its proboscis.
Exploder: unlocked at level 7. Although very weak, the Exploder can be a powerhouse if used correctly. Its attack is its signature kamikaze explosion, able to hit multiple enemies if hit correctly. Having its pustule dismembered will be an instant-kill.
Stalker: unlocked at level 16. Just like in the campaign, the Stalker's main advantage is its use of flanking hit-n-run tactics. Its special ability is hugging a wall and being able to peek around the corner to see if there are any enemies. Its main attack is its frontal charge.
Those are my ideas so far.
I honestly don't see how this couldn't have been done. The overwhelming variety could be a problem, but this could be solved by having a load-out screen similar to the security team's, where you pick the four variants you want to use. Maybe they'll do this in the next game.