Even if Bethesda's at top form they're still going to struggle to recapture Skyrim's popularity, because it had a great hook for mass appeal. For starters, everybody loves dragons, and considering that they were technically a new addition to the series as well, Bethesda had all the ammo they needed to put them front and center and hype the shit out of them (I distinctly remember lore purists being adamant even up to a week before Skyrim's announcement that dragons were long dead and gone from Tamriel and were
never
coming back). What can ES 6 deliver that's more hype-worthy than finally bringing dragons into the playable games? Outside of doing something drastic like contriving a reason to bring back the Dwemer, I can't really think of anything.
Shouts were an interesting hook as well - a simple, unique, and memorable ability that gave warrior-types easy access to magical abilities and encouraged exploration far better than any of the game's standard loot (standard loot being half-assed in Skyrim even by Bethesda standards). What new ability could Bethesda introduce in ES 6 that tops or even matches Shouts? Can't put that genie in a bottle and limit the player to just the standard fighter / mage / stealth skills of Morrowind and Oblivion. I always thought it would be cool to have the ES 6 protag be a Dragon Breaker, where they can warp time / space in small amounts to create combat effects (and the chief antagonist could be a supremely powerful Thalmor Dragon Breaker who performs grand feats like restoring the Crystal Tower by 'stealing' it from another timeline, which could have unforeseen consequences like weakening the planar barriers and allowing some non-Dagon, non-Molag Bal daedra an opportunity to invade Tamriel).
I also feel like the heavy Norse themes came about at just the right time when people were getting sick of the more generic Central Europe-type fantasy settings.
Simple questions deserve long-winded answers that no one will bother to read.