I count at least one date with Squidward/Squilvia as canon, as it was implied strongly in the episode. If they stayed together after that, obviously we don't know. It bites that we've never seen her again, honestly.
Now, obviously, since I very seriously ship Squidiam (Squidward/Squilliam), I'm fine if she isn't going to end up as Squidward's long term girlfriend. - I'll also admit, though, that even if she would be, it wouldn't stop me shipping Squidiam. It'd make it a bit trickier to fit my head-canon with show-canon, admittedly, but I'd be up for the challenge. ;) I would like to know more about her: Hobbies, interests, would she have actually ordered at the Krusty Krab (something not really seen with octopus characters, sans the one realistically drawn mom with her children in "Sailor Mouth", etc.. Actually, I wouldn't mind seeing her interact with Squilliam somehow (I do NOT want him to take her from Squidward, though, if the writers actually did that, I'd be ticked.. I can't see her being Squilliam's type, and Squidward just does not deserve that). Based on her personality so far, I don't think she'd be interested in Squilliam either. She liked Squidward for cussing out idiots among other things, while we've only ever seen Squilliam being snappy/mean/rude towards Squidward (he's nice/friendly towards literally every other character he interacts with).
Oh, for the record: Back to the Past is the last episode we see Squilliam in, actually. But "Keep Bikini Bottom Beautiful" was his last larger role.
Snellie's another character I wish we'd have seen again. I don't really like the idea of Squidward ordering a pet just to enter her in a race and then getting rid of her after that. Plus, I adored her and Gary.
Spot's another character I'd like to see again, too. I've only seen the episode once, but I remember liking it a lot.