It would be great if there was an episode where the Patty Vault is burgled (by Patrick, but Mr. Krabs would think it's Plankton), and SpongeBob references the deleted scene by mentioning that they used to have a burglar alarm, but they had to remove it. That would be hilarious!
I think that Gorilla should returned, because that part was so freaking out of the ordinary that's it so funny at the same time. And also that future references of Spongetron. Oh, and Patrick Squarepants.
A lot has been said about the Tattletale Strangler and I agree there should be more straight villains in the series, however I'd much prefer Dennis to return. He is a typical hitman character, but they've done archetypes well before in the show, and due to season 9b/ten/whatever you wanna call it pulling off plots quite a bit better than the past, I believe Dennis would work well as a main villain for at least one episode. In addition, a Dennis/Karen team-up might work. If there's another themed miniseries in season ten, I hope for Dennis to reappear. Why in a miniseries you ask? Well he wouldn't fit in a random episode of modern SpongeBob imo, so maybe a themed miniseries would be the best fit for him.