We expect the Protocol Droid parts to stick around awhile right?
Yes, the folks from Disney Parks were very excited to be expanding out into protocol droids and it showed when they spoke during their panel at SWCO.
They indicated that they want to have a constant presence of both Astromech and Protocol droids.
My thinking is that they will get into a groove of new waves of parts every 2-3 years. We had parts in 2012, 2015 and now 2017. There have been straggler parts, like for example, some new heads were added on to the 2015 parts around the time that TFA came out, plus they have thrown the holiday edition hats into the mix from the 2015 & 2016 droids.
So if I were to hazard a guess - new parts will be coming in 2019 to coincide with the opening of the Star Wars lands at both Disney Land and Disney World.
Next year will be nothing but carded figures, only now instead of it being all astromechs, I think there will be carded protocol droids released for certain events - I don't see any reason why they wouldn't consider having protocol droid exclusives for May the 4th, D23, Not So Scary Halloween or Xmas in 2018. And of course, don't forget that BB-8 as a Build-a-Droid comes out this summer around the time of the D23 convention, so there's always a chance that different versions of the BB unit would come out as well - since they once did a Holo R2, since Holo BB-8 shows up in Star Tours, that's my vote for the 2nd BB unit release.