My premise here is that canonized ships are priority in terms of being produced. Above that, ships that both fit the timeline of the galactic civil war, and also appear on screen, are the highest priority right now. This gives Rebels an insane advantage in terms of ships.
Imperials however, are down to one final ship. The Quasar. And even THAT is used mostly be the rebels since they captured it.
So where can we go from here?
A lot of us expect the Imperials to get the "MC-80" treatment by having a 2nd Imperial Star Destroyer expansion, with the ISD-1 physical model and a white paint job, to reflect the ISD's seen in Rogue One. There were even a few new Imperial leader names thrown around in that movie too, to fill in several characters and new commanders.
Thrawn might have more ships in the 7th fleet... and if so we can expect them to show up when Rebels kicks back in to the 2nd half of season 3... but I'd expect any new Imperial Ships not to show up until the season is nearly over. I expect the season finale to be a crushing defeat for the rebellion, and this would be a perfect time for Thrawn to show up with some new Star Destroyer.
So this gives us 3 possible top-tier options. The Quasar, an ISD1 repackage, and a possible new ship from Thrawn's 7th Fleet. (Big assumption here, since we have zero evidence that he is indeed hiding a new ship).
So what else is beyond this? Well the Finalizer of course, from The Force Awakens, but that is likely going to need to wait until Episode 8 comes out, because the Resistance will need some sort of counter to it. We also have a few new classes of Star Destroyer mentioned in the book "Catalyst", such as a Kaz class, and even a vague reference to a Dreadnaught that was finished in just one year.
Could the Dreadnaught be code for the Executor? I'd doubt that they built a Super Star Destroyer in just a single year. Dreadnought can be such a generic term, describing the classic dreadnaughts from the Katana fleet, or just an extremely large ship in general.
Maybe even Tarkin's Carrion Spike, it could be a ship with just one title, and have the title give some type of cloaking technology - perhaps all attacks are obstructed... or maybe a special type of obstruction, like remove TWO dice.
However my prediction is that the next ship to come out for Imperials AFTER the quasar, (or in addition to the Quasar, if we get a 4-ship wave) will be the Venator. Reason being, is that you see it on screen in Rebels, behind Tarkin during the 1st episode of season 3. According to the books there are still some Venators in service during the timeline of Rebels, plus it would be a huge fan service.
What do you think we are likely to see for the Imperial Ships?