So, what sort of enemy clubs do you guys want to see Ryuuko face? I can think of a few fun ones:
Swimming: We've yet to see how Senketsu reacts to being wet, which could interfere with the blood in his fibers. At the very least, a goofy battle in a pool seems appropriate for the show.
Bug-Catching: A no-brainer in terms of entertainment value, especially given how many over-the-top bugs exist in Japan. Think a high school version of Tommyrod or The Pain from Metal Gear.
American Football: As a team sport, it would be cool to see Ryuuko forced to assemble a ramshackle team consisting of Mako and her family in order to play by the rules. That, and it doesn't take much to imagine Trigger animating some insane tackles.
Paranormal Club: Ghost/Alien themed opponents could present unique and unpredictable challenges to Ryuuko. Okay, that was a generic justification, but at least you can expect jokes on probing/ectoplasm.
Manga Club: If Trigger is feeling really ballsy, which they always are, they could attempt a fight scene done entirely in the "animated manga" style showcased a few times in FLCL.