In my eyes, as Blaze implies, Alton Towers is in need of both a starter coaster and an indoor coaster; not desperately, but ideally. Where my opinion differs from his is in that I think that they should definitely be separate attractions. I've seen little-to-few examples of indoor kids coasters, and I'm not convinced that they work, nor are they worth the expense which goes into them. Sure, Alton Towers can stick a Mack YoungStar or a Troublesome Trucks clone somewhere on park; Beastie site, Dungheap site, Coaster Corner, Boneshaker / UG Swinger site, there are certainly options, but that's an entirely separate debate. Much in the same vain as that Alton Towers is in need of more filler 'flat' rides.
As for the indoor rollercoaster, well, ideally it should be well themed and a family-thrill attraction. Although, I admit, I'm rather easy to please, I'd be delighted with something of Raptor Attack standard, even if it's worthless in terms of track and train, because you can't go wrong with an 'experience' coaster, something which I feel that Alton Towers has lacked since the removal of the Black Hole. Ideally, for me, the ride type would be that of Eurosat, but Sonic Spinball prevents this from being a plausible idea as far as park management are concerned, so I would hopefully suggest a Gerstulaer Bobsled. You never know, it might already be in the works, with Alton Towers getting a deal on an indoor Bobsled & SW7 from Gertsulaer... wishful thinking, I know.
Now, although I've never ridden a ride of this type (to my knowledge), it hardly takes a genius to see that Van Helsing's Factory is a work of sheer genius, one which I'm eager to ride. If Alton Towers could pull off a similar standard of layout, combined with the high quality theming which seems
to be becoming more likely, looking at 'Raptor' and 'The Swarm', along with the high quality surroundings planned for SW7, then they could quickly be on to a winner.
With regards to location, I'd be reluctant to say the Air car park as many others here already have, but I'm honestly struggling for other potential locations for such a large scale dark coaster, unless Duel or Charlie & The Chocolate Factory (neither of which would be exceptional losses to the park line-up) were to be removed, so I've been caught out there. Indeed, I find the Black Hole tent site to have been realistic for such a project, but that has now been taken, for the greater good it could be argued.
In short, yes, I agree that Alton Towers need an indoor coaster in order to compete with the likes of Disney and Europa Park, but they're still a long way off of that target. Then again, it would make a dramatic difference to the quality of the park's winter line-up if an indoor coaster could be opened, too, so it would be of obvious benefit to the park in that respect.