Pokemon is genuinely the one theme I am so surprised has not been exploited more in the western world. Great shout.
After a quick think, the following are theme's I'd like to experience, in order of preference from 3rd to 1st (they may exist, I don't know about them if they do)
3) Minecraft, it's such an iconic game that has stood the test of time so well, unusual for 21st century creations. and it's a theme you could do a lot with. Mine cart coaster, rowing boat splash cart / log flume, a nether dark ride, a, erm, twirling toadstool ?
2) Discworld... A crazy fantasy theme that you could have a lot of fun with.
1) Middle Earth / LoTR... Should need no explanation. This would HAVE to be done by Disney or Universal on a Pandora / wizarding world scale though to do it justice, and it's just not going to happen.
OOPs a special mention for a game that got finally released last night, and is awesome... Cyberpunk 2077's world would make an amazing themed land... Not sure that kind of content is suitable though ? P.s for anybody interested, the game is also playing very well so far.