I've always been intrigued by the possibility of fantastic space travel, i.e. traveling to other worlds by means of magic or non-scientific/technological methods. I suppose I like Jack Vance's "Morreion" a little too much or something. In principle, I thought TSR's Spelljammer could have been cool, but, as it turned out, it was just silly. In my Dwimmermount game, for example, it's well established that elves are the scions of the otherworldly Eld, whose home planet of Areon once had more regular contact with the campaign world. The same is also implied about the Green Planet, Kythirea, but there's much less direct evidence of it. Nevertheless, "interplanetary" travel is a fact, even if it's a somewhat obscure one to all but the most knowledgeable sages.
Now, this hasn't yet come up in the game, but there's a distinct possibility it might in the future. Dwimmermount is full of gates to other places and (again) it's a known, if obscure, fact that the Eld invaded the world to acquire its azoth, large quantities of which can be found within the ancient mountain-fortress. Azoth is, of course, the liquid form of ether, which fills the space between the worlds. So I'm giving some thought on how I might handle space travel when it comes up and how it meshes with the overall cosmology implied by OD&D magic.
Once you add Supplement I, you get the Ethereal and Astral Planes and they both work somewhat like and somewhat differently than the way I'm conceiving of "outer space." That means I need to consider whether to include those two planes as written, eliminate them, or modify them. Each of those decisions has consequences; my goal is to think about those consequences beforehand, so that I'm not caught unprepared later.