I've always imagined some ingame level select menu that keeps track of Time, Kills, Secrets, Items, etc. It'd have cute little thumbnails and descriptions for each level of each official IWAD. Maybe some hardcoded exceptions for levels where percentages break (via Archviles or Icon of Sin, inaccessible items or secrets, etc.)
I always liked the idea of doing a 100% run of whichever Doom game, but the transient nature of Doom saves don't have the same impact to me as something like 2016/Eternal, or even something like SRB2's emblems. I wonder how possible this would be to do in Zscript, at the very least.
Maybe include something like Cutmanmike's Map Order Shuffle, or something like WadSmoosh being built into the port itself. (I've seen some ports support dual-loading 1 and 2's assets, plus NRFTL or SIGIL, but it'd be neat supporting Every official WAD at once)