I really wish the Heart of Lorkhan wasn't destroyed - it would have been really cool if it was in that cube north of Winterhold and not just the Oghma Infinium. I've been thinking of playing Morrowind sometime soon - just gotta bother quickly finding a working, cracked torrent of it - to generally play more Elder Scrolls, but to also perhaps learn more about the Numidium and the Heart.
Given the Falmer live underground, it's quite possible they could proliferate - in a widely-organised way - throughout most of Skyrim.
So far as other realms go, checking out Hermaus Mora's Apocrypha could be really cool. All that juicy forbidden knowledge~
Using a Dragon Break could work well to, at the very least, establish a concrete path for the Dragonborn, paving the way for events in the subsequent roleplay. It could have occurred because, say, the Dragonborn fucked up in some way, and Akatosh initiated the Break and chose some kind of agent to act through who would travel through time during the events of TES5 to correct 'mistakes' made by the Dragonborn, this Agent of Akatosh also possibly being a very important character.
Stemming off from that, it could be interesting to maybe have some unmalformed Snow Elves brought back (as roleplayers), who could choose to try and help their deformed kin or eliminate them and restart with a 'pure' race of Falmer.
Again from the Dragon Break, perhaps Apocrypha might need to be reached in order to understand why the Dragon Break occurred, what it changed, and how/if anything of it needs to be changed.
Messing with alternate realities lalala~
How do they sound?