Ooh, good thread, I bet there's a ton of interesting ideas out there, I know I've got loads I'll never do anything with, since I thought of writing for tits before I got sidetracked into doing fan art (and then again into other artistic pursuits).
Personally I'd want to write a Nova/Goo Suit expansion that adds the option for Nova to fight at your side like an attack drone.
I'd love to see a Nova expansion in general, she's fun from what we get of her, and I feel like there's a ton of potentially fun stories to be told with the grey goo and Captain Morrow in general, and this would be a clever progression hook to tie it to.
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A lot of the ideas for characters I had (I drew a bunch in my art thread) involved playing around with the ideas of gender and social convention as set up in-universe. So like, a submissive Dzaan alpha - might that be equivalent to being gay (and maybe taboo) in their society? Or maybe an Anat female who rejects her privileged but constrained role at home, and runs away to find freedom in the rush (this would have been a rival-like series of encounters).
I also had a few ideas for races, like a fully aquatic species who can psionically manipulate the water immediately around them, both allowing them to get around and for water tentacles and other such kinky stuff.
Following the TiTs tradition of drawing from examples of the weird real-world sexual behaviour of animals, I was inspired by parthenogenetic lizards to create a scaly all-female species that reproduces asexually, giving birth to more of themselves, but still engages in ritualised sexual behaviour with other females to induce pregnancy. I planned to give them tentacle hair or crests and then I realised I had literally made the Asari from ME. This happens to me all the time.
The silliest idea I had might have been for a race that always gives birth to 'twins', but due to high levels of Savicite exposure on their planet they actually become psychically linked into a single mind (not just telepathic, but actually an individual with two bodies). I came up with a bunch of stuff, like reproduction, and how to deal with one dying or separation or whatever but that was the basic idea.
I'm sure there were lots more but I have a feeling most people won't wanna read too many at once so I'll stop for now
Oh, and if anyone is inspired by any element of these whatsoever, go nuts, I'm cool with that.