Think it's unlikely Castro is back in 2016. Cubs can take pose, as Craig posits, that don't have to trade him...but, as practical matter, awkward for everybody to keep him as a utility guy going forward. Castro has been good trooper for tail end of this season but wouldn't assume that for 2016.
I expect Castro's future depends on what other teams offer. Two extreme possibilities:
1. "no-talent-return": Cubs are so happy to offload Castro's salary and the awkwardness that they'll trade him without expecting any meaningful talent .
2. "great-talent-return": Some teams think Castro is a terrific value, and will offer really good talent to get him.
IN between "no-talent-return" and "great-talent-return" extremes is a vast middle ground. My guess is that many shoppers will be interested in Castro only at cut-rate price, based on his poor hitting two of least three years, and on a perception many GM's may hold that he's soft/dumb/distracted. If the offers aren't very strong at all, Cubs may or may not sell, depending on how they themselves evaluate him. I'm not sure whether the "awkwardness" is that big of a drive; Castro is a mild personality, eager to please, dwarfed by the bigger/stronger personalities in the Cubs clubhouse. I'm not predicting they won't trade him, but I can very easily envision them not doing so. Designate Schwarber and Baez as the LF/2B starters; Coghlan and Castro formally "subs" who could play a ton and enable Maddon to adapt/adjust to almost any and every situation.
More broadly, don't think we'd see all the current positional shuffling if Soler was healthy. Maddon has gone from a stifling 4-man bench to an 8-man bench. He loves that, fun for him, and keeps the regulars fresh. But tougher to do with the usual 4-man bench to open 2016.
I think a 12-man staff and a bench with Ross, Castro, LaStella, Coghlan, and Denorfia could be exceptionally functional. Maddon could still flex a lot, platoon, mix-and-match, pinch hit. Perhaps replace one or more of those guys with somebody with similar profile, or better. Of course, that's assuming that Castro could hit when he got used, and that perhaps LaStella could too.