I don’t think there necessarily a perfect formula for it. Arguably Alton Towers is becoming roller coaster heavy, having removed The Flume and replaced Charlie with an upcharge Dungeons. Of course, water rides and dark rides can also be headline rides. Clearly people do like variety, and there can be other downsides with being very coaster centric. Coasters and water rides are more likely to exclude guests with physical disabilities compared to things like dark rides or shows, and dark rides often have lower height requirements (although not necessarily). Alton Towers doesn’t have loads of rides for people who are under 1.2 metres, but who are too old for C Beebies Land. Dark rides and shows are also more likely to be undercover, which gives people something to do when the weather’s bad. Although again, not necessarily. And the Merlin parks don’t tend to have undercover queues for their dark rides, which negates one of their advantages. A few do, like Hex.
The thing about Alton Towers (and some of the other British parks) is that on a peak day, there can be virtually nothing with a waiting time under 30 minutes. If you go to Magic Kingdom on a peak day, the queue for rides like Seven Dwarfs Mine Train, Space Mountain and Splash Mountain will be very long. But even on a peak day Magic Kingdom tends to have attractions with short queues, like Carousel of Progress, Hall of Presidents, Monsters Inc Laugh Floor, Country Bears Jamboree etc. Even on a peak day you can go to Europa Park and go on something like Madame Freudenreich or Geisterschloss without waiting. At the UK Merlin parks, on a busy day you end up queuing 30 minutes for something very basic.
Parks like Disney, Efteling, Europa and Phantasialand don’t have loads of flat rides, although there are other kinds of support attraction. I don’t think Alton Towers necessarily needs loads of flat rides, although they clearly do get a lot of complaints about the queues, so removing flat rides like Ripsaw or Submission and not replacing them with anything else, is going to make the problem worse. Arguably what they really need is a couple of support rides that have a throughput of 1,400 people an hour plus. That’s part of what helps to manage the queues at places like Disney, Universal and Europa Park.
These kinds of rides don't necessarily come up if you ask people about their favourite attractions. But things like the omnimovers at Europa Park do absorb a lot of crowd. At Disney visual presentations like 4D cinemas, animatronic shows, circle visions etc can be a bit dull (not all of them), but they do achieve big throughputs at a low cost.
