I think any of the second tier prospects (those considered to be below Baez, Alcantara, Bryant, Soler, Schwarber and Almora) especially those at AA or higher, will be candidates for add-ins on trades this winter and spring.
By the way, CubsDen reports that there has been a lot of talk on twitter saying that Raul Alcantara of the A's will be the PTBNL in the trade. That doesn't make a lot of sense since he has two more options left and recently had TJ surgery. But I suppose it could be accurate.