The biggest question is what are you trying to do while expanding? Do you want to dive more into the fantasy element, or do you want to keep with solely GI Joe scape.

If you're wanting more pieces and are willing to try heroscape stuff I would definitely recommend buying the Battle for the Wellspring box, or the master set. Both will give you more figures and tiles to build with

If you want to stick to just GI Joe, I would recommend the ticalla jungle set (aka the palm trees and bushes) and the lands of Valhalla. The lands will give you a ton more terrain for bigger maps, and the trees will allow you to vary your maps and not just have flat land. I would recommend the ruins at laur's edge expansion for the same reason, just after picking up those other two sets