Assuming they don't just double dip on Ancestral Steppe (which I think would be in the running), my guesses would be for
Misty Peaks from Tri/Portable 3rd,
Jurassic Frontier from Generations Ultimate, or
Ancient Forest from World. Smash leans very heavily on the 'first area' and those three are the ones that fans of Generations (Ultimate) and World would experience first and get the most exposure to and are from the bigger (biggest in the case of World) releases of the franchise. All three also have varied locales that can be used for differentiating environments like caves, open fields, (destroyed) camp sites and so on. Outside of those three, there's also
Deserted Island from Tri and
Elder's Recess from World that they could choose as they both have reasons to be considered (first area of Tri that introduces many of its mechanics, a major area in World where three of the big fights take place), though I'm not as confidant in those two.
In terms of what monsters would appear, Those like Zinogre, Rathian, and Deviljho are givens, they're extremely popular/recurring monsters and they would probably not miss the chance to have them as hazards to cart Smashers with. The rest depend, but if they don't care about which games they appear in then those like Lagiacrus, Mitsuzine, Nergigante, Gore Magala, and Valstrax are also good candidates to appear.