Well touch wood, in 10 years time we should see the 'priper' Thorpe Park Hotel open and fully complete, along with an entrance redesign and no more Thorpe Shark Hotel.
So taking that into account, we'll definitely see a more resort feel to the place I reckon. Which means they'll need a major family thrill coaster. Just look at how Towers needed Th13teen back in 2010, and how well that's been received in the long term. So I fully expect one of them. Who knows when it comes to manufacturer though...
Also, I wonder how long Colossus will have left come 2026? It'll be 24 years old and I fear it'll just get worse with age. And with the likes of the other 4 major coasters we currently have, plus the potential for two more by 2026, will Colossus really be maintaining its popularity. I really doubt it. If we ignore the sentimentality behind the ride, and how important it was for getting Thorpe 'on the map', it's just going to be an uncomfortable, old ride with a gimmick which has lost its importance. Space will also be a premium.
Straying away from major developments, I'm a bit worried about the park's filler rides. Merlin seem to not like adding in flat rides to their parks (have any of their parks actually seen a brand new, outdoor, flat since they took over?), and Thorpe have a lot of flats which will have ended their life in the next 10 years. Heck, some of them will probably end their life in the next 2 years. So what will happen there? They can't just add dark ride after dark ride, and they can't remove rides and not replace them. And even some of their water rides need redevelopment; Loggers does, Rumba will, Tidal should. There's a lot to do aside from major developments.
I've rambled a bit here... But basically, we know in theory there should be a hotel, potentially 2 new major big rides. One of them needs to be a family attraction. One of them could be an RMC or some woodie, given the park lacks that. But I don't think that will be the most interesting thing to happen in the next 10 years, but instead everything else that happens in between.