I've got a few ideas, though this list is far from exhaustive and a few topics are probably obvious.
Introductions (for ... introductions)
Table Talk (discussion from a player / enthusiast / GM perspective)
- Recommendations
- Looking for groups / players
- Questions / Advice
- Rules Discussions
Design Discussion (discussion for people making / hacking / designing games)
- General theory discussion
- Advice and critique
- Shameless Self Promotion
- Hacking discussion (for hacks of other systems: PBTA, Forged in the Dark, etc.)
- Cons and such (I hear cons are important ... I never get to go to them, but I guess people should have a space to talk and maybe plan meetups)
- Playtesting (looking for playtesters, best practices, playtest notes, etc.)
- Shop talk (for discussion of marketing, sales, boring and awful business crap)
- Opportunities (tips on PAID work for people with game design and game design adjacent skillsets).
Art and Stuff
Again, these are just a few ideas. Honestly, I am not sure if this many topics would be necessary to start off. When the community is still small and new, spreading things out too much may not be the best idea? But I do think general discussion, player discussion, and designer discussion sub-forums may be a good place to start.