I have been looking into the original source materials for what we have seen thus far.
Here's how it breaks down.
Excluding the starter set, I count 19 expansion releases so far (all known releases up through wave 4).
As one would assume the majority of the ships (13 of 19, 68%) have come from the original trilogy.
Of the 6 remaining, one was originally found in a book, three were originally seen in video games, and two (or three, depending on how you count Imperial Aces repaints) came to us from comic books.
Z-95 (Han Solo at Star's End book, plus WEG RPG material, etc)
The HWK-290 (Dark Forces video game), the TIE Defender (TIE Fighter video game) and TIE Phantom (Rebel Assault 2 video game).
Imperial Aces is from the comics, as far as I can see (the 181st and Emperor's Guard squadrons came from Dark Horse Comics; specifically X-Wing Rogue Squadron and Crimson Empire), as is the E-Wing (Dark Empire comics).
So, I assume we will have to see a group of items coming up that hail from that multi-platform release (book, video game, and comics): Shadows of the Empire. In many ways one could claim that Shadows of the Empire has a stronger canon claim than the other non-OT stuff released thus far.
Probably the Outrider and Prince Xizor's ship, at a minimum, would be released, but possibly as much as Black Sun becoming a third faction could be in the works as a method for bringing this property into the game.
I would prefer bounty hunters and Black Sun remain "Imperial" rather than making a third faction, as it keeps the game simpler.