What about Puerto Rico? It's a bit poor, but I think it would be a good size tropical island with elevation and a good road system.
Edit: The more I look at Puerto Rico the more I think it would be a perfect island for TDU
Honestly, what bugs me the most about TDU 2 are the engine sounds. They are for the most part just straight up horrible and non-accurate. Have yet to find a decent tutorial on how to make your own sound mod for TDU 2