I've loved D&D for many reasons, including the ability to change planes and campaign settings via plane shift, gate, and other means of interplanar/interdimensional travel. (BG2EE has portals to other planes, extraplanar creatures, a Planar Sphere, and a Spelljammer!) I wished Planescape: Torment Enhanced Edition added adventures on other planes, not merely relying on the content from the original version.
Big mods exist already for EET and Icewind Dale-in-EET is coming Soon™, but I'd like to know in a low/no spoiler fashion what mods exist - if any - that grant extraplanar adventures.
As for my favorite planes and extraplanar places to visit, I'd like to visit the Great Wheel, the Outlands, and the Planescape setting in general beyond what's in PS:T. I'd like to try Krynn, Ravenloft, Athas/Dark Sun, and Eberron to name a few.
Finally, to clarify, I don't mean just to 'play the game' as it is and experience the extraplanar adventures. I've already done that.