I asked this question on Twitter yesterday - I'm looking to improve my own Patreon-campaign - and the most common answers were as follows:
Q&A-sessions with the creator and the characters themselves.
Exclusive livestreams only accessible to Patreon-backers (which I am looking into setting up, but I might need to hit another milestone before it happens: I'd need a premium membership on Picarto)
Early access to upcoming pages so that backers are able to read ahead of regular, non-backer readers.
WIPs, behind-the-scenes and concept art posts so that backers get a peek at the process behind your work, and can either learn some fun trivia, or learn a few tips and tricks to apply to their own drawing process.
Monthly sketchbook PDFs emailed out to backers on the relevant tiers, with images not posted elsewhere.
I'm unfortunately not in a financial place where I can afford to back people on a regular basis, but if I could, I think I would most like access to the sketches and concept art and behind-the-scenes-stuff, the Q&A-sessions and early pages.