The problem is X:\ is a prime example of how to have a good coaster, but theme it so badly that nobody know what its supposed to be and the main response is it is a load of rubbish.
Give X:\ a good theme, even put a little smoke and some good lighting effects in there and it would be good.
For me you could easily have a haunted house type theme going on:
Just imagine as your going up the lift hill backwards, suddenly a large spider lights up and the fangs head straight in from of you.
If an indoor coaster has a good theme, it is well done, then there is no reason why it can't work. As JW said about the wild mouse in the dark, check out this: http://www.youtube.com/watch?v=nwLtwrUL7PI
One area I thought this could work is at Katanga Canyon. Call it something like "The gold mine" enclose this ride it a pretend rock face (simular design to the RMT at Chessington but bigger) with the existing RMT.
The existing RMT could be under ground on the large turn before it goes into the rapids tunnel, add some good mine theming like you see in Indiana Jones or James bond, some mine sounds and maybe some lighting with flickers. by enclosing it in a tunnel, it will make the ride seem a lot faster and it would be a mine train, rather than a mine themed train.
Then the wild mouse, if the same model as the Rattlesnake at Chessington could have the cart go up the lift hill and the first two turns of the mouse can be in the open air with the quests in the queue line seeing the cars moving on top of the mine, then the remaining turns go into a tunnel, where it is dark and the rest of the ride is completely dark.
Just an idea I came up with, I'm sure Alton can get a cheap second hand Wild Mouse from somewhere and add some fake rocks around the outside.
I know its not indoor, but most of it can be indoors and the queueline can be set up where you go down the mine tunnels to get to the main station.