I'd like to see more themed flats like Ripsaw because they're good fun, don't cost much and are always great just to watch. But I'd also like to see more dark rides.
Something like 'HP & the Forbidden Journey' or 'Spiderman' would be good too. But as Nightfall said, they just don't have the same pulling-power or intimidating visual impact as a great big coaster whizzing past full of screaming riders.
But there's always the option for parks in the UK to combine the two. A new coaster grabs everyone's attention and the dark section ticks the box for a lack of great experience rides. Even if it's only a small dark section. Saw is a good example. As is Th13teen (even if the actual coaster section isn't brilliant). Perhaps, if and when Drayton Manor build their next coaster (2014?) they will incorporate a short 'Blue Fire' style dark section before the main coaster bit? Anything to add to the overall enjoyment and uniqueness of a coaster gets my vote, as does good theming.