If they do bring this, I'd like to see:
1. An option to pay for it per day, not have it automatically be the length of the ticket. Unlikely, I realize.
1b. If not paying per day, it would be nice if it were a flat fee and not dependant on the length of ticket, the way that park hopper and water parks & more are. so, that you can add it on for (say) $40 per ticket whether you have 3 day tickets, 5 day tickets or 10 day tickets. It would give greater incentive for longer stays, like the other options do.
2. Have the additional FP+ (3 or whatever) be usable in a different park than the "base" 3 FP+.
3. Allow for doubling up on attractions with the additional FP+ -- being able to use them on the same ride as another FP+
4. No tiers for the extra FP+ -- usable on any ride with FP+ capability.
#3 and 4 to me are the key ones to make them work. If I can buy FP+ that let me go on Soarin' and TT and allow me to ride TSMM multiple times in a day, etc. then I'm in for a reasonable cost.