If anything, we could see 8-bit Deathmatch of other franchises in future, such as Kirby, Metroid, Zelda, Castlevania (Though this would be insane due to the sheer amount of weapons and skills existing through all of them)... Not sure if the MM8BDM team would be willing to do that, for they would still be as time-consuming as this one, but I'm playing with possibilities.
I'm gonna stop you right there and say that is a set of bloody awful ideas.
Kirby would take a lot of work to make the copy-ability function properly, Metroid is best done with detail on the environment (8-bit doesn't have much detail, really), Zelda isn't an adventure game and deserves more depth than deathmatch (actually, same goes for Metroid since it also is based on exploration), and Castlevania isn't much without some creatures of the night to whip to death.
I'm surprised Mega Man works, all the rest would be a bloody mess to implement to a deathmatch setting. Maybe some deathmatch to the side, but not as the main course. Especially since you'll be most likely destroying many aspects that we all know and love from them.
Besides, there's already some mods based off some of those for Doom. Metroid Dreadnought, the Unnamed Zelda Mod, and Castledoom/Doomvania.
Excuse me, but I can prove you wrong in regards to Zelda and Metroid. Metroid got two or three Prime games with DM elements, one of which has two kinds of enemies use bot AI actively, and they are as annoying (if not more) as Zandronum Bots. As for Zelda, Link Between Worlds provide some sort of PvP, not sure if bot-based.
But in regards to Kirby you are MOSTLY right, however Kirby Triple Deluxe features a Smash Bros-like PVP (except it is health based instead of sending everyone flying off), so that kinda gives some hope.
I was also going to mention Mario or Pokémon, but I can't see how the former could work nor how the later could be stable and even scripted (absurd amount of codes).
Oh, don't put much money on the ones for Doom. There has been previous approaches at adventure-oriented Metroid and Zelda FPS for Doom or similar (Custom Game Maker Engine) that never went on.
Also, consider: Mega Man's inventory is larger than all those I suggested sans Castlevania (which hold a monsterous inventory)