I hope that the creators of this game are takings notes about this ideas, it's really wonderful that they take our opinion on futures sets, greettings

B-wing

TIE Bomber

Skipray Blasboat

YT-2400

That's my guess.  

People seem to be forgetting the canon story setting of Shadows of the empire which have at least 3 ships they could use outside of the B-Wing and TIE Bomber. 

The Star Viper attack platform which was developed by Mandalorian engineers and mass produced after the fall of Xizor.  Then you have the Aggressor assault fighter popularised by the IG-88 and finally you have the YT-2400 (although this is the least likely to be added of the three)

Wave 3 is going to be revealed this year as it doesn't make sense to stop making models when you have as lucrative a licence as Star Wars.  FFG are going hell bent with a three pronged star wars product line all of which can be expanded upon multiple times, so cancelling one of these lines after wave 2 would be pointless and only give the licence up to another company to profit from it, since miniature wargaming is very popular.

But let's look at the situation with wave 3 as it's a little tricky.  We have no solid information about what era FFG are going for with this game.  We know they have acknowledged the expanded universe with some of the pilots and other references but that isn't proof that we will see things like T-Wings anytime soon.  But we have 2 movie ships left to add, the B-Wing and TIE Bomber which we can assume are definitely in wave 3.

So what we have left are two slots for this wave.  The Z-95 is now canon after it was added to the clone wars series recently, so technically it can be added to the rebel faction.  The V-Wing could potentially be added to the Imperial faction as it was used during the jedi purge era after revenge of the sith, but it may not stack up compared to other imperal vessels.  One thing you have to remember is that imperial ships like the TIE series are cheap for mass production purposes so they aren't necessarily better than clone wars ships, they are just easier to make in numbers and require less maintenance.  This could allow ships like the V-Wing to be used as a tougher fighter that is more survivable, but less maneuverable.  Another possibility is the Sienaar developed Skipray, but this is never in the movies and until we know exactly what era's FFG are planning to use it has be counted out of any future waves.

The shadows of the empire could be used for the aggressor and Star viper.  IG88 could be a imperial unit since like Boba fett he works for the empire when needed.  The star viper is tougher to give a faction to, so maybe it wouldn't make the cut.  This leaves the YT-2400 which could be another large vessel that like the YT-1300 can be a neutral vessel used by both sides for different scenarios.

If they decide on a clone wars era expansion, then it should be in a new core set with new rules for droid fighters and the like.  It seems like the most logical step since there are plenty of ships to choose from this era compared to the original trilogy.

On the point of ground vehicles I have to ask why?  The game is called X-Wing and involves dogfighting, unless there is a variant of the AT-AT with sublight engines =p.  Ground battles need a separate game and a civil war era ground warfare game would be tiny and massively imbalanced.  Using only the three films for reference the rebels have turrets, ion cannons, air speeders and soldiers.  The Empire has massive armoured walkers and soldiers, the Empire wins every single time.

Ground warfare would be better suited to the clone wars anyway since there are again more vehicles to choose from here and both sides are more balanced, but I don't see FFG getting into the ground warfare wargaming when they have space combat, a card game and a tabletop roleplaying system to print money from.