I'd like to see a general tropical theme, but a new one that wasn't in TMS. Maybe a jungle

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A south american jungle with jaguars and tapirs and aardvarks and... trees. Although probably only the last one of those is possible XD. Or maybe a south-asian jungle like Adabat in Sonic Unleashed. But Adabat looked a little too much like island...
Of course, and I've said this before, the environment I'd really like to see is a volcano. Not a cheap volcano mod but an actual volcano environment.