IP's are, nearly always, a proven success. As are 'gimmicks'. Maybe not to enthusiasts, but to the GP, they lap it all up - which is why most other successful parks copy that approach.
I think whoever replaces him has a huge opportunity in the next few years to really attack the UK market - lots of people who used to travel overseas will likely look to spend their money domestically, and that pent up demand will be looking for days out and short breaks that appeal to them. Expect as much focus on the 'resort' aspect as the theme park itself.
Just as a general conversation point here. IPs certainly can be a success. But how many of Merlin's IP choices have been a success? There's certainly a fair few which are up for debate...
How to Train Your Dragon
Kung Fu Panda
Room on the Broom
All of those I'd argue have not be successful, or at least not as successful as they should have been. And there's more
Yes, you've got the likes of Saw, CBeebies, Gruffalo, perhaps Walliams World and Jumanji which have done well. But there's just such a hit and miss nature with IPs, especially when you look at the successes of, for example, Wicker Man and Smiler.
There's a place for IPs, definitely, but I think Merlin's heavy leaning into them in recent years has harmed them more than helped them.