Kid Buu is almost a given I'd say for the next villain. Considering how Tamashii has said they'll be concentrating on the Buu saga characters.
Here's the confirmed line-up:
OCTOBER
-Android 17
DECEMBER
-Legendary SSJ Broly
FEBRUARY/MARCH (Estimate)
-Android 16
MAY/JUNE (Estimate)
-Vegito
I'd say the remaining 2015 line-up chances would look something like this, in no particular order:
-Kid Buu
-Majin (Fat) Buu (If they do another Buu, I see him before any others...it might also depend on Broly sales)
-Buu Saga Vegeta (Majin & Regular heads and faces)
-Gotenks (with various heads/faces)
-Ultimate/Mystic Gohan (VERY easy for them to make, hopefully includes SSJ head,sword and some action accessories)
Re-releases, who would serve nicely to break things up and go along with new releases:
-Trunks
-Teenage Gohan (manga ver. with extra face and battle damage arm?)
-SSJ3 Goku (Better paints? More matte and less glossy)
I think Vegeta might be ruled out because they'll want to do a Buu Saga version. Plus we saw a lot of Vegeta with two releases in 2014. A newer "Cell Saga" Vegeta might be in line for 2016.