EurActiv has reported that Viviane Reding, the current Justice Commissioner, may be positioning herslf to run as the European People's Party candidate during the 2014 European Elections. She's in her third term as a Commissioner (I've heard it said that Luxembourg keeps sending her to Brussels to remove Junker's biggest party rival from the home stage), so she's very experienced when it comes to the EU and it's executive wing. Reding has also served as Commissioner for education and culture in 1999-2004 and as Commissioner for information society and the media from 2004-2010.
EurActiv comments that she had a base in the EP, which could serve her well in gaining the nomination. I'm not sure how well known she would be in the Member States come election time, however.
The possible challenger from the Party of European Socialists is current EP president Martin Schulz. The PES will hold primaries in the run up to the elections to choose a candidate, but timetabling issues (it looks like the primaries may run partially during the winter holidays) may mean that early positioning is an effective way to ward off competition that could struggle to gain momentum. He's not held a post in the Commission - I don't think he's held an executive post yet apart from being mayor of Wuerselen - but he has been an active MEP and served as the leader of the Socialists and Democrats Group in the EP before becoming the EP President.
So neither candidate has a continent-wide profile in the same way that national heads of government have, but a good contest could quickly change that, particularly if the elections become a referendum on the economic direction of Europe. Both Reding and Schulz are able to give good speeches, so a head to head debate could inject more energy into the elections (certainly, they're more engaging than Barroso).
I can't help wondering how the other Europarties are going to react - will they run their own candidates, or will they try to gain policy concessions in exchange for parliamentary support (and would that mean that we'd effectively be voting for coalitions in the Parliament)...?