Yes, actually, the on-going development for the update will include many very minor revisions, and much like that of the development updates for Porsche cars, the parts for the new editions in many cases, will be retrofit compatible.
Some will be handling improvements or retune for a different taste (of which in stock form will not be so usefully different since most of the people here are more akin to more drastic customization) but if things like lengths of lower arms, uppers, links or sub-frame are changed, it will be nice for us to combine new and old to align things to personal tastes more...
Because of the market shifts and restyle needs for refresh, cosmetics will also be addressed. One in particular, which isn't related to USA but in Japan, the front marker lamp being placed really low on the bumper for the 86/FRS makes lowering the car "technically" illegal in home market. While this is not a huge issue for the authorities who tend to not give much attention there, it is more a classification and ruling for many of the grassroots activities there like Gymkhana, where Stock Class regulations need to be street legal. BRZ has a turn signal in the front lamp assembly so they have been fine, while the 86 was an issue, so this might be revised so that the 86 will have new headlights to incorporate and address that.
Some parameter changes that will not add power but for CEL trigger parameter to be wider ranged and not so sensitive. Most of the CEL triggers are caused for stipulations and overly critical Emissions standards of the USA, and triggered by random occurrences that really have nothing to do with operation of the vehicle. So some of these might be addressed.
We've seen 18% fatter drive-shafts on the Subaru BRZ tS editions to improve rigidity of the rear axles, and indirectly also improves shift linkage feel.
To also credit Subaru, the production facilities will get a infusion of better assembly robots to replace a lot of the human labor that's been unusually costly for the BRZ/FRS being so drastically different from the Forester and XV Cross Trek that are built on the same line. So we should expect similar pricing despite the improvements in many areas.
Power upgrade? (I can't tell you but time will tell.
Steering rack bushing will also be updated, for firmer response with track compound tires. (it won't make much of a difference for users of street tires though, since it's pretty fast and responsive already.)
So those are the likely items we might see in the next update.