Honestly, if done correctly, a park shouldn't need anything outside of it's own intellectual property. You can have a fun, refreshing themed environment without IP. It just takes some creativity. A park has no limits when they aren't strapped to a licence agreement.
Plus if you dig around, there are some iconic characters that are actually part of the public domain that could be harnessed.
Like a park that no longer sells merch for DarKastle, Grogan's Pub, Mach Tower, LotD, and the huge list of other rides or attractions for which "souvenir" merch is possible. Don't get me started on the years of wasted opportunity not marketing and selling Celtic Fyre or any other shows, except that London Rocks garbage. Instead, we get generic park shirts and crap that you could buy at any park. You couldn't tell except it has "Busch Garden" printed on it. But, how many people bought a shirt for a ride that isn't even there anymore? I see BBW merch practically disappear when they put it out. They definitely need a marketing class on how to leverage the attractions and brand they had...I mean have.