Alduin didn't die because his soul didn't get absorbed by The Last Dragonborn, so there will be a sequel or a DLC.
Alduin isnt dead, you didnt get a soul from him but thats probly because hes a Demi God (Akatosh's son) so its probly not going to be so easy for a random dragon born into a mans body to simply kill him and take his soul. He's probly going to make a return but for the time being i doubt the game will be going back to skyrim right after we finished it up, that be boring as hell. their going to take us somewhere else probly one of the Dominion controlled countries (Summerset, Elswyer, Valenwood) seeing as they are rising in power now and we havent visited them since Arena.
I wish they could put all of Nirn's continents as the main quest for TES VI, so we could visit where has none has gone before.
^they have ES online coming, im sure Akavir will be a subject of the expansion and possibly the islands to the south were the Sea Elves live too. but if you mean one huge detailed single player game like skyrim then thats impossible they would need, with current technology, like 8 install discs or for consoles a disc for each country you visit. would be awesome but i dont see it happening :(
It will happen in the future... when the virtual reality or something similar has been made[Imagine... your in-game NPC wife and you, doing things(training with swords, making a child, etc)].