I personally think a story about the Dwemer would be great. I loved the "Lost to the Ages" quest in Skyrim. Dwemer are so shrouded in mystery, so I'd want to know more about them.

we have an entire thread dedicated to TES6, you should post your suggestions there

http://www.gamesas.com/topic/1496217-official-beyond-skyrim-tes-vi-34/

oh, sorry, I haven't been on this website in a long time. so I'm kinda new. Thanks!