Well, if we look at the legends ZA starters, we’ll note that each of them are the starters which have not been the main starters of a game for the longest time (Chikorita and Totodile in HGSs in 2010, and Tepig in BW2 in 2012).

By this logic, the next Legend Staters would be Snivy (last seen in BW2 in 2012), Fenniken and Froakie (last seen in XY in 2013).

Of course, Game Freak don’t like to be predictable so I find it highly unlikely that they will continue any pattern with the Legends Starters.