I can think of two types of big thrill coaster that would fit the park perfectly atm. An RMC inverting woodie behind the Swarm and an Intamin Hyper, possibly stretching across the front half of the island in conjunction with a new entrance area similar to what Cedar Fair has been doing recently. I chose Intamin over B&M because they are better suited to twisted layouts, they don't have to be built as high to get a fantastic ride and because I freakin love Expedition GeForce.
I also agree that a smaller coaster between major investments is a good idea. A powersplash sounds about right although I'm not sure if it would necessarily fill a gap in the park's ride lineup, since they already have both a launch and a ride with a big boat, if that makes any sense. I don't think that'd be too much of an issue though, just a random thought.
A smaller coaster I would like to see is an S&S 4D Freespin. As long as they can fix the capacity issue, I think that it could work wonders for the park as it's a new concept to the UK, has a lower height restriction than most of Thorpe's current major rides and is a good size for a park that is admittedly running low on space, not to mention they are reportedly fantastic rides. Plus, I think it's different enough from the Swarm for both rides to stand on their own despite the similar seating arrangement.
1. Helix | 2. Taron | 3. Nemesis | 4. Expedition GeForce | 5. Wodan