If the team follows the same strategy as they did before (one racing driver + 1 rookie driver all season), I bet it's between Jeroen Bleekemolen, Carlo van Dam and Giedo van der Garde.
Giedo van der Garde and Jeroen Bleekemolen are very likely to be available for A1GP from the start of the season; Giedo is in the world series by Renault, Bleekemolen in the Porsche Supercup and both can win their titles without missing out on any A1GP race / test.
Carlo van Dam races in the Japanese F3 series, can also become the championship winner but the last race of the season overlaps with the race in Mugello (any idea about the Macau GP?)
All other drivers have other priorities:
Robert Doornbos is in Superleague Formula, very likely to be followed by Yelmer Buurman (him testing is already announced); their races overlap with the start of the A1GP season.
Christijan Albers is in DTM, while 2 of the 4 remaining races overlap with A1GP races. The same goes for Renger van der Zande and Henki Waldschmidt who are in the F3 Euroseries (with top-3 chances for Renger).
Junior Stous and Arie Luyendijk are in the Atlantic series, with at least 2 remaining races overlapping with A1GP. I don't know about their title chances.
Jos Verstappen messed up with Jan Lammers and Ho Pin Tung already raced for A1 Team China so both are unavailable.
http://a1.gpgrid.com