Urriak Uruk
Gaming is fun, and fun is for everyone
That is the reason I mention so many times the chronomancers and time spheres, because allowing a D&D version of Marvel What If? or DC Elseworlds would be nice.
Thing is, D&D is already a "What if?" People take stuff and change them at their own table. Your homebrew world, or your version of Forgotten Realms at your table, is your Elseworld. If you want chronomancers or time spheres, use them.
The difference is you're asking for official Elseworlds/time spheres, which is more likely to p*ss off "lore purists" and long-time gamers than be attractive to people who like crossovers.
Personally, I find crossovers kind of silly and only really work for super-heroes. It's something I don't really need to see too often in D&D; the odd mention of another world, or character from somewhere else passing into another is fine, but I don't want to see Strahd, Asmodeus and Acererak team up and make a Legion of Doom for the heroes of D&D to unite against. I find that pretty off-putting and more of a marketing gimmick than actual compelling story-telling.