Supreme Court case - Supposing they find that the proroguing was illegal and therefore didn't happen, as you were, parliament recalled. Given that it didn't really work for Johnson anyway (Benn Bill still went through) what is to stop this:
Remainy gang still don't want GE before 31st Oct, so Johnson limps on. Pretends a deal is getting closer. Comes back from the summit mid Oct - soz, no deal happening. Prorogues parliament again on 24th Oct, just for a week and resigns so he doesn't have to hand in the extension request. Then we're out, GE happens and he rides back on a crest of Brexit party supporters.