As long as you don't go all scumbag like everyone else seems to on DA "uncensored version on Patreon!" "Full picture only on Patreon!" Paywall-treon is just dumb. I mean, it's fine when it's things like extra stuff like line art, or tutorials and bonus stuff, but when artists limit half to 3/4 of their content behind a Patreon paywall, it's just scummy.
"I know you guys have been following my art for years and I wouldn't be where I am without your support... but fuck you, if you're not paying for Patreon you don't get to see any new stuff." is kinda how I feel most people are treating Patreon.
I'd say like tutorials for patrons, psd files (which it seems you already do both of) maybe like skype chat sessions where people can ask you questions and discuss art for patrons only and such, variants of certain pictures. Basically anything that isn't totally screwing over all the people who can't pay for whatever reason, by restricting the content they get. But those are my 2 cents, I'm sure people taking the scumbag greed way are making shit tons of money by screwing over their long time fans, and sadly they seem to be okay with that.