Honestly, there’s a lot of great answers in this thread. Personally, I’d love to see a FOSS ttrpg manager. Talking a complete library of monsters, races, classes, etc., along with an optimized pipeline for homebrewed stuff. Tools for encounter, battlemap, NPC and campaign flow creation.
Closest thing is 5e Companion App but it doesn’t have a PC client, isn’t FOSS, has a lot of weird limitations and UX/UI issues (like multiclassing could be simpler, and its really frustrating that you can’t level down a character after all the work you did, forcing you to do it all over again just to change classes and spells). Also DnD next but getting source books for a whole player session is expensive.