It needs to be some kind of water transit ride, not necessarily a flume but something that relies on water to move you around. They really need at least one ride like this asides from the rapids to keep some variety in the parks line up, now that more or less every flat has disappeared from the park.
Otherwise what will Alton Towers be? 10 coasters, CBeebies Land and, well, not much else.
They need a water coaster. Basically. But more of a water ride than a coaster. Or a pirate version of Chiapas - fun, well themed and not too wet.
This is really the time when the park need to get Merlin to provide the funding for serious investment over the coming years - Alton Towers need a major family water ride, dark ride and coaster before the end of the decade if they want any chance of getting back on top.
I've said this before but they need another 1992 and 1994 imminently - pity there isn't the vision that there was back then.