No SRD at this time (to be honest, I wasn't expecting interest in that sort of thing, but now I have a reason to think about it).
It's using the Creative Commons Attribution 4.0 License, so things are pretty fair game. I don't want pro-fascist or bigoted content made for it, but based on your other content I don't see that being an issue (just stating it because this is also a public space). I don't want the original game in its entirety getting sold without my permission.
As far as types of content, the sky's the limit! Adventures, additional mutations, additional gear, random roll tables for scrap in specific areas, maps, monsters, notable Oheys, cybernetic rules, survival systems, nest-building systems, hacks, fan-fiction and things I haven't thought of are things I'd be delighted to see. Feel free to charge for whatever you make. I think community copies are cool.
I'd say keep the main characters squirrels, keep mechanics pretty lightweight, and try to keep it so d8s are the only dice at the table. The other fundamental piece would probably to keep the tone light but the danger and stakes high. That's not to say horror or political thriller aren't options, but remember the game is being run by a Nut Master and write accordingly.