Current list of things I'd like to see added/expanded on. Don't take this as a promise that I plan to do them in any timely fashion or a territorial stake that I'd rather do them than let someone else (if someone can add these without my needing to make the effort, all the better really, I am rarely around).
- Comprehensive list of early Doom WADs which were broken by later vanilla releases of the game (that is to say, WADs that were reliant on pre-v1.9 behaviors). A well-known one would be the boss behaviors in Doomsday of UAC but there were also some WADs which were affected by changes to sector movement code (i.e. the stairway trick in VILLAGE.WAD). The latest version that sector-movement-related ones work in is usually v1.2 as far as I'm aware. Most likely this would only be very early releases for the original Doom, with no Doom II WADs being affected by such issues?
- What would be a good category name for this?
- It might be worth checking GZDoom codebase for leads as I've heard several special compatibility modes exist that are triggered only when certain WADs are detected.
- More coverage for Engine bugs in Heretic.
- Compare Heretic source release to Doom source release to figure out what changed in line-of-sight calculations as a probable cause for Monsters can see through walls.
- More coverage for Engine bugs in Hexen.
- If someone wants to fill them out, check this page which has some example maps/demos showcasing various anomalous behaviors: 
- Pages for the unique H!ZONE episodes/levels (the map/walkthrough/analytics treatments).
- I'm pretty sure the unique Hexen levels are conversions from the Heretic episodes so check and confirm that too, probably. If so, that can probably just be noted on the pages for the Heretic maps when they exist?
- Info for emulating DOS front-end programs like D!, UltraLaunch, etc. These rely on functionality of DOS that DOSBox doesn't correctly emulate last I checked; one solution is to run 4DOS in DOSBox before running the frontend program. A VM/emulator that actually runs a full DOS install, such as PCem, would probably also work.