What do we know is left open? Several things, and maybe they could be tailored to Play Styles-
The 'Aldmeri Question'. Could be interesting to explore Aldmeri intents, from either side. Might be a good stealth character DLC, or for the warrior type that tries to solve questions with an axe to get introduced to the need for stealth. Could be interesting for the alchemist or mage
Lots of "redo this, redo that" stuff and I in general hate that, but how about revisiting Solstheim? No need (or want, from me) to revisit 'Bloodmoon', there's a lot of possibilities here, in a larger and more detailed Solstheim, and it also invites underwater content, being an island. Dunmer influence should abound, making it much different from what MW players recall. Based on what we know about events after Red Mountain, what has happened to Solstheim seems to be a great opportunity. Perhaps Solstheim wants independence, maybe as a Dunmer nation. Could be a great DLC for the agent/thief.
How about a Challenge to the College Guildmaster? Yup, you became Archmage really easily. How about DLC that puts you through a challenge from an Archmage from another province, who is angry such a neophyte runs the College of Winterhold (even if you're an expert, he still assumes you're a nobody)? You'd have a series of quests that require you to be, in actuality, the expert mage you're supposed to be. Maybe a magic-oriented 'Trials of Hercules' concept
But easily the best idea I can come up with is this- rescuing the Souls that Alduin 'consumed'. Alduin was un-made, or destroyed, but what of the souls he harvested? Lots of possibilities here, that range from Tomb delving to acquire a bone from the fallen hero to use as a focus for easing his lost soul's passage back to Sovengarde to journeying to planes of Oblivion to treat with Daedric Princes that seized the souls that where aimlessly journeying after Alduin was defeated. Maybe the Souls are spread out over Skyrim and Oblivion, trapped into weapons and armor by enchanters that didn't realize (or who did realize) what they were, and the enchanters are using them for their own ends. Only disenchating them to speak to the Soul and then following a Quest to send them back to Sovengarde could restore them to their rest. Maybe Alduin's soul, his body destroyed, is in one of those weapons or sets of armor, and has possessed it's owner, making a man or mer that is master of the Voice that you must defeat. Maybe that possesed person is a necromancer who is trying to harvest the souls himself, and is creating a nice undead army. Warrior type is obviously the most typical answer, but mage could also be a great choice if done right
I'm sure there are dozens of other possibilities



