This is such a hard guess, because Universal's FP+ system is so different. It includes all (participating) rides, either for one ride on each ride, or unlimited depending how much you want to spend, no reservations necessary. This honestly sounds like a much better system, though for us, would be absolutely unaffordable.
I honestly don't see them doing the exact same thing as Universal, so I think it'll be something different. My guess, they try and work with the system they already have and do something like:
Off-Site: no FP+ included, must buy. Purchase 3 for X$ and then add additional for Y$ each.
On-Site: depending on what level you are, you'll get FP+ included (maybe 1 for value, 2 for mod, 3 for deluxe) with the option to purchase for more a set price. I could see the price per extra FP+ being less for the upper levels.
I can also see them allowing better booking times, so similar to how it is now: off-site 30 days, value 45 days, mod 60 days, deluxe 75-90 days etc...