Fastpass + is in testing, but nothing more than testing at this point.
There are rumors for how it may affect Guest Assistance Cards, mostly in terms of using an RFID enabled card, bracelet or some other device to become the GAC.
That would provide more security and allow it to actually be 'tied' to the person it was issued to.
Those, and anything else, are just rumors at this point. Actually, there is very little besides rumors about how Fastpass Plus will actually work.
When I know more, I will update post 6 of the disABILITIES FAQs thread.