I wonder if they will reopen the fort to guests? Probably not.
Will everyone PLEASE stop hyperventilating about Fantasmic! with zero understanding of how it actually works? I have friends who work Fan as talent, techs, and costuming. There are several facilities that support the show backstage:
The Fort is largely used for supplies used to stage the show, including pop-ups, tables and chairs, low-level lighting, canvas or plastic walls along the Island's waterline, props, etc. Generally things that don't need to be cleaned or repaired on a regular basis (or transported back and forth).
The Marina was the home of the Princess Barges; the support boat that towed the pyro barges, pushed Columbia when the engine is disabled (the show can't run without Columbia, but can without MarkTwain), and general support on the river; and TicTock the late crocodile. It's also where they load the three pyro barges before towing them in front of the stage.
The main Entertainment Building behind Toontown, known as N-19, is home to tech services and costuming.
At this time it is not known exactly what will be housed in the new Fantasmic! building behind the RR track, other than boat storage. What is known is that it will be larger than the old marina and have improved facilities. But it is unlikely that it will supplant any storage in the Fort because everything would have to be transported back and forth to the Island. Logically, it could include a small pyro storage locker, sufficient for one day's supply of shells.
The new metal building being completed in the area MiceChat identified as Expansion Area is designated in the City Permit as wardrobe and dressing area. One might assume that some of this space will be for Fantasmic! costume storage. This would replace space at N-19, which is at a premium with the shifting lineup of parades.
Nothing indicates that these facilities will significantly change use of the Fort or return it to guest usage.
If anyone has more hard data about the return of Fantasmic!, by all means chime in. But everything I have heard suggests that storage for the show is not being appreciably diminished on the Island. Rerouting the river and the train wasn't to benefit Fantasmic!--it was for Star Wars Land. Any benefit to Fan is a byproduct.