The Imperial Assault releases follow a pretty basic pattern based on moving chronologically through the original trilogy.
A New Hope - Big box, the starter set. Small box, Tatooine.
Empire Strikes Back - Big box, Hoth. Small box, Bespin.
Return of the Jedi - Big box, Jabba's palace. Small box, Endor.
So there are a couple of different options for after that, I'd be curious to see which people think will be most likely/most exciting.
Force Awakens
Pros; It keeps moving chronologically forwards. It keeps the original trilogy aesthetic that all of Imperial Assault has. The movie was crazy popular and models for Rey, Finn etc could possibly bring new people into the game.
Cons; Big box is obviously Jakku, but no obvious small box expansion. Once in the new era players might be reluctant to move backwards, which could put FFG in a tricky situation where they have to wait for new movies to develop new content. Details about the state of the galaxy in this era are actually pretty sparse, might be better to work from an era with more details to develop from.
Rebels
Pros; The Inquisitor has already been released, so FFG are clearly looking at this. Keeps the original trilogy aesthetic. Good collection of villains to use (Agent Kallus, the Inquisitors, Maul, Thrawn) as well as the rebels crew themselves. Lothal would be a good location for a box expansion. Tons of details and ideas to develop from. AHSOKA.
Cons; Not as popular with older fans. Moving backwards chronologically.
The Prequel Era
Pros; Um
No but seriously there are tons of cool soldier types and designs as well as fun locations, and the mass war would give FFG infinite possibilities to play with. And the prequels are under rated there I said it.
Cons; Would be moving backwards chronologically. Different aesthetic. Difficult to balance against the generally lower power level of the other eras. Could be a huge nerd backlash against it. Sand gets everywhere.
I'm not counting Rogue One as an option, just because there isn't enough time for FFG to develop for as their next set of releases after Endor. It does look awesome though and I'd love to see it get some Imperial Assault love down the line.
Wild card option, FFG drops the release convention for a year or so
Pros; Not limited in what box expansions can be released.
Cons; Could lead to a weird mishmash of different timeframes etc.
On balance I think a focus on Rebels for two expansions, before moving onto Force Awakens era, would be best. But I'm curious what other people think.