Hmm... There's been a lot of different ones that I've thought about with different levels of seriousness. So much so I can't really remember them all. Some of the ones I do remember:
Head Survival 2 - Basically, advanced version of the last one, which was more of a jokewad. Take advantage of all the latest ZDoom features, expand the way battles progress, more classes, etc. Maybe get some other forum goers who are willing to donate their faces.
The main reason this won't be going anywhere is a reason that pretty much covers everything I don't do: there's too many other things I want to do more than this.
Zombies Ate My Neighbors - Not a conversion, but a similarly set up style of game, since ZDoom supports just about everything you would need. Co-op support included.
Same reason as the last one.
Déjà Vu - The familiar is unexpected. Basically it starts with a normal map. Each map after that is the same thing but just a little bit different in increasingly bizarre and unexpected ways. As you feel like you are moving through a familiar environment, you in fact know nothing about it.
Same reason.
Some Kind of Scary or Maybe Otherwise Frightening Wad - Assemble a small dedicated team to try and make something that is genuinely scary, but not like a Ghoul's Forest clone. Something a bit more involved. I have ideas here and there about some things, but being a team project it would require some sitting down and discussing what everyone wants.
Same reason as the others, but this time also because I don't think I could drum up the support needed.
BTFDSCM: Boy, The Freshly Dead Sure Can Move - Multiplayer competitive game where all the players are zombies and there are crowds of living, breathing NPCs to slaughter. Compete for points by killing NPCs in various ways and do other things to try and interfere with other players.
The usual reason.
Minigame wad - A collection of minigames. I imagine it would be similar in concept to Turbocharged Arcade, though I wouldn't know as I still haven't played it yet.
Same reason.
I know there's dozens of other ones I'm not remembering but that's how it goes. I don't really come up with them because I plan to ever work on them.

