It would seem that standalone streaming of in-market MLB games is going to happen next year one way or another. If those rights go to individual teams, rather than the overall league, then teams will each strike their own deals for standalone streaming access. That could happen a number of different ways: partner up with Amazon, Hulu, YouTube or another digital outlet for distribution; allow the RSN that holds their broadcast TV rights to also sell in-market streaming subscriptions; or simply use the existing MLB.TV service to handle in-market streaming subscriptions for their games.
If, on the other hand, in-market streaming rights stay with the league rather than go to the individual teams, I would expect the league to either renew their streaming deal with the Fox (now Diamond) RSNs or bring the whole thing in-house and sell both out-of-market and in-market streaming subscriptions through MLB.TV.