Science Fiction, amateur radio, electronics... and maybe some equivalent of the popular (popcorn) subreddits; damnthatsinteresting, wtf, showerthoughts, TIL.
Ultimately I don't know yet. I'm hoping to see some new stuff that I didn't think to look for. Any topic can be interesting if people are passionate and not toxic.
This. I participate in homebrewcomputers on reddit, would be nice to see something like that here, although the website is small enough that posting diy computer stuff in "Open Source" would probably be acceptable.