-There are like 6 different Spider-Woman outfits from MARVEL they could take from similarly to how they took from Xena and Wonder Woman for Sivir.
-Mechanical spider legs and armored outfit would be cool. Some sort of mecha-spider transformation with spider drone minions.
-Some sort of furry/fluffy spider. A lot of spiders out there are actually very hairy such as tarantulas and wolf spiders.
Elise happens to be humanoid and transform into a spider. She could easily have a number of human outfits. Could have a dress that flows with spider form. Could have a Rockette outfit; they have a number of outfits that are viable and it'd fit nicely with the dance. Some sort of female tuxedo could also work, if they gave the spider a tophat and tuxedo-colored/covered body.