The arena would make a great place for a dark ride, one side is covered by X and trees anyway, and that site is also quite hidden, its also a very large site so you could fit something in there.
I don't think a dark ride would work on the beach because its the entrance to the park, and building something there would just block the whole park, and probably confuse you even more as you enter the park. But I agree that a nice entrance area would be great there, they could also remove the whole dome and start again, but that would be very expensive, and unless they build some rides in to it, then I can't see them gaining many visitors (that is what Merlin will be bothered about). Also if they do replace the dome they will need a bar, shop, arcade, coffee shop, guest services, toilets, lockers, first aid and what ever else is built in there (offices I guess?). If not they could put some smaller flat rides or childrens rides on the beach area.