I'd say Tamira Paszek has a chance. Her ranking really masks the fact that she had an atrocious year before her surprise Eastbourne win/Wimbledon defence. There's a lot of points that are there to be made up, and she's not exactly a million miles away from the top 20 as it is.
Ula has got less than 300 points to defend until the end of May. Her draws are going to get easier too with her good ranking. Top-20 is quite possible.
Robson? Probably, but I find her abhorrently annoying.
Jankovic Just kidding, wishful thinking.
Ula, Hsieh, Barthel, Stephens (???), Begu, Arruabarrena-Vecino, Watson, and Pervak all strike me as very strong picks for Top 20 (given none get injured).
For me Ula ( #15) Hsieh ( #18) and Robson ( #20) ... Ula has her chances in the beggining of a season .. she has nothing to defend ... maybe 2nd round of AO