Here is some thoughts on games which OBLIGE may and may not support in the future, and the reasons behind it.
DOOM 1, 2 : works very well
FINAL DOOM : working well, could use their new textures some more
HERETIC : currently non-functional, but won't be hard to get it working again
FREEDOOM : is not a selectable game in Oblige, but since it is compatible with the DOOM games you can just build maps for DOOM and they should work fine in FreeDoom.
HEXEN : it was working in the distant past, but needs a lot of work to revive it, and need to support Hexen format prefabs.
HARMONY : a nice-looking free game that uses the DOOM engine. A basic definition was written long ago, it would need a lot more work though.
HACX : another free games using the DOOM engine. There was some support for it already, but it would need a major update.
CHEX QUEST 3 : a more expanded game than the original two, which uses ZDoom, and hence may require Hexen format to achieve things seen within the game. I personally have never played this. Enhas made a start on a game definition but it needs a lot more work.
QUAKE 1, 2 : these were once supported (in V4), but Oblige has changed the way it handle prefabs since then, storing them in WAD files for easier editing. But wad files are too limiting for Quake, so would need another way to create prefabs (ideally could load them from ".map" files).
DUKE NUKEM 3D : a cool game that I like and have recently added support for the map format to the EXE (C++) code, although it is not complete yet. It currently has a minimal game definition and needs a lot of work, including adding information on monsters, weapons and pickups which are currently missing.
STRIFE : another game I haven't played. Uses the same map format as DOOM, hence only needs a game definition file. From what I've heard, this game has an involved plot and I'm not sure the gameplay is suitable for random generation. Engine has a hub system and that will need extra logic (like Hexen).
AMULETS & ARMOR : this is a RPG which uses the same level format as DOOM. I don't know if the engine crashes if too many lines or sectors are visible, which would be a deal-breaker for Oblige.
HALF LIFE : the original Half Life uses an engine called "GoldSrc" which is based on the Quake engine. Hence if Quake get supported again (with the features mentioned above), then supporting this should at least be possible. One concern is having to create the AI pathfinding nodes.
DARK FORCES : I played the demo and thought it was pretty cool. I've also read a paper where somebody used this game as the target of a random map generator. The map format seems similar to Build, so I think generating maps from OBLIGE should be feasible. I don't know if the engine has limits (e.g. too many visible sectors crashing the game), as that would be showstopper, unless a better engine was the target.
BLOOD and SHADOW WARRIOR : I have never played these games, however they both use the Build engine, and hence if/when Duke Nukem 3D support reaches a decent state then it should be possible to create maps for these games. I have no idea whether their gameplay is suitable for random generation.
HEXEN 2 : this uses the same map format as normal QUAKE, so if that game is supported then this will "merely" need a game definition for all the monsters, weapons, pickups and themes. I have only played the demo, so I'm not sure yet if the gameplay is fully suited to random generation.
SERIOUS SAM 1 : The recent release of the SS1 engine source code means that supporting that game is now a lot more feasible. It would require heaps of work though.
In general, anything released after the year 2000 or so is too modern for Oblige, the map formats become too complicated for Oblige to create them.
WOLF3D : Wolfenstein-3D was a game I played a lot when younger. It was briefly supported in one of the V2 releases, though it did a pretty poor job, but at least the game definitions (monsters, weapons etc) are there. The map format is extremely limited and I find I get nauseous when playing this game nowadays.
BLAKESTONE and ROTT : these won't be supported for the same reasons as Wolf3D : the architecture is too limited (would need separate logic in the Lua code for it), and I don't enjoy playing them.
CHEX QUEST 1, 2 : I feel that these are obsolete with the existence of Chex Quest 3 (which contains all the resources from the original games).
MARATHON 1, 2 : these games are completely free (you can legally download them), so I'd like to support them even though I don't enjoy playing them. However the original engine has low render limits which crash the game if overflowed, as well as a limit on number of objects, and these limits have not been removed by the AlephOne developers.
QUAKE 3 ARENA : this game, and the completely free version called Open Arena (which I have played a lot), are multiplayer games. These kinds of maps (Deathmatch and CTF) are less suitable for random generation, an opinion I explain in the OBLIGE FAQ, so I don't plan to support them at this time. Supporting the format may be possible (via q3map2), though the logic for using .map files as prefabs is not there yet. Hence any other games using the Quake 3 engine is also excluded.
HERETIC 2 : this Raven game apparently uses the Quake II map format, assuming that is true (or any difference is small) then supporting it would be possible. The game has a lot of story to it, so I wonder if this game is suitable for random generation.
THIEF : I haven't played this game, but did a bit of research on it. There is no source code to this game, which makes it much harder to create maps for it (though the OpenDarkEngine project may be helpful). I don't even know if it's possible. Also the engine may be too limited, perhaps crashing if too many areas are visible, and that is a show-stopper for Oblige support. Lastly I'm not sure the nature of the gameplay, where you must often sneak around and prevent yourself from being seen, is suitable for random generation -- it would definitely require a lot of extra logic.
CUBE : this is a freeware multiplayer game. The map format is a bit limited (using a grid of 16x16 blocks), but could be doable. While it does have monsters, there is too much missing in the engine to make decent single player levels: no doors, no keys, not even a way to progress from one level to the next (AFAIK). The successor SAUERBRATEN might be more suitable....
DOOM 3 : this uses a .map format which is similar to Quake mapping, however the tools for building the maps are contained in the game EXE, and this is a show-stopper for Oblige (which would need stand-alone tools).