I personally would probably go with one of the pre-defined plot integral characters, such as Almalexia or, if a player character was needed, Cyrus. For the most part, major side characters could do anything the heroes could do, as well as potentially having plenty of background to themselves that could give them things that maybe the player can't. Cyrus is pretty notable as well for being a major figure in Tamriel's history that the player actually gets to control and has a preset look, personality, and voice. It'd also be nice to have Michael Mack again, after the apparent absence of his voice in Skyrim.
The reason I avoid main characters is it seems like it'd be a disservice to make it the Dovahkiin, Nerevarine, or Champion of Cyrodiil when, even though they had "pre-defined" appearances in promotional artwork/trailers, hardly anyone actually ended up having their character look like that. The only player character from the main series that comes to mind as semi-usable is The Agent, but that's probably from personal experience where every single one of my Agents ended up http://i.imgur.com/WkrT9.png for a brief period in their lifetime, with so little showing of the character that even a major thing like race could be a pallet swap away. This being TES though, even if somehow everyone's character did look like that briefly, it'd be lame to represent a series that often has many players with completely different characters represented by someone who's supposed to be THEIR hero who looks the same as everyone else's.
But that's just what came to mind while I was mulling it over earlier today. What do you think about it?