The problem with Pandemic: Legacy is that it still suffers from the fact that the core mechanics are based on Pandemic.
As for IA, I think we can look to Descent 2E for a good estimation of where it is going. Now, obviously Descent 2E was a little skewed because it also re-implemented material from the fist edition of the game in the form of the Hero and Monster packs. But if we take those out, Descent 2E has had (so far) 2 Big Box expansions (which each added a full campaign) 5 small box expansions (the last two can be combined to create one full campaign) and a number of lieutenant packs that were sold in conjunction with each boxed expansion. There was also a separate campaign book that was sold in a hard back edition for owners of the original core set (the more recent core sets having the original campaign replaced with this new campaign). Then we have the RtL App which has given then entire product range a fresh pair of legs without FFG having to release a third edition. So, the way I see it, we will get a few more independent blister backs (probably based on characters from the OT and Rebels) and at least one more small box based on Endor and maybe another small box or even big box based on Rogue One. I think FFG will save any material related to the prequel and/or sequel trilogy for a separate core product (call it second edition if you want).
What I am most interested in, as a campaign only player, is, no surprise, more campaign material. Either in the form of stand alone campaign books or hopefully in the form of an app. I think after two more boxes, we will have enough tiles and figures to support years worth of new campaigns. And maybe FFG will release a vault web app like they did for Descent 2E which lets us create own own missions/skirmishes/campaigns. Of course, if FFG really is making an app, they might release more material just to keep that going as well.
I suppose we will find out soon enough though as to what is in store for IA in the near future....