Generally, I think the purpose of using an IP can be summarised as providing an attraction, that on its own merit doesn't offer a compelling enough proposition that's also marketable, with a hook that will (hopefully) draw in the crowds. To use the examples Dan referenced above: Angry Birds Land and I'm a Celebrity are both pretty standard attractions, which would probably have come across as quite lame if they didn't have the respective IP and had to be marketed as such, and although DBGT offers a much more bespoke experience, it would have been much harder to convey the sort of thing it involved without the Derren Brown association. I suppose you can also consider X's TWD retheme - without the IP (which has also enabled the broader Year of TWD offering), a retheme would have likely been a waste of resources from a commercial standpoint.
Where as, when it comes to installing a high-thrill rollercoaster, it tends to be much easier to market the ride experience for what it fundamentally is. Of course a complementary theme also helps to increase its marketability, but the theme being that of an IP won't necessarily make a significant difference. I think it's for this reason that none of Merlin's major rollercoasters, with the obvious exception of Saw, have utilised an external IP.
Therefore, I do think it's more likely we'll see something original for Project 2020, although of course that's not a certainty. Ultimately, it will depend on how impressive the ride experience is that they can come up with.