An interesting theory I've heard that I'm inclined to believe is that some kind of new iteration of Palpatine's personal flagship, the Eclipse, will be making an appearance. The Eclipse stands alongside things like the TIE Defender as one of the better ship designs that the Expanded Universe introduced. They did a really good job making it look visually distinct from the rest of the Star Destroyer line while still looking like it belongs within the original Star Destroyer line.
A big issue I have with the Sequel trilogy is that it feels like it's afraid to introduce new ship designs for their Star Fighters. Keeping the X-wing line around makes sense given that X-wings are probably the most highly accomplished and versatile ship designs in the Star Wars universe. After all, it was an X-wing that destroyed the first Death Star. I do wish they made a few more tweaks with the design though to show that some time has passed.
The T-85 X-wing from Disney's EU sort of tweaks the design of what we saw from the resistance X-wing enough to really make it feel fresh compared to the X-wings we saw in the original trilogy. I feel like the A-wings in TLJ should have been replaced with a completely original design though.
The First Order on the other hand has no excuse. In legends the Empire was actively replacing their old TIE fighters with the superior TIE interceptors. The interceptor is just better than the TIE Fighter. End of story. What they really should have done is used the TIE Defender. It's an absolute juggernaut of a ship that looks incredible from a visual standpoint. (it looks a bit awkward in still images but when you see it in motion it makes sense). It also would have meant the TIE Defender subplot from Rebels would have actually gone somewhere in the overall saga instead of being completely contained within that series because the Empire is too cheap to make the investment necessary to mass produce them in any capacity despite the fact that the Defender is obscenely overpowered for it's time as far as Star Fighters are concerned.
linkAt least though the Capital Ships generally look nice and distinct from their OT counterparts, outside of the Raddus and even than the Mon Calamari cruiser got woefully little screen time in Episode 6 and always kind of felt weirdly abstract (how do they launch fighters for example) which is something that The Last Jedi did a good job of really fleshing that design in a way I really appreciate. It makes sense that the Resistance is going to be scraping the bottom of the barrel as far as ship designs go. With the new republic fleet mostly wiped out and the Resistance fleet now consisting entirely of a single YT-3000 light freighter as of the end of TLJ there really isn't much left as far as newer ships and designs are concerned that the Resistance can call for help.
Instead what we saw from the trailer with that fleet shot is presumably a bunch of planetary defense fleets and whoever else the Resistance could convince to join in banding together. What appears to be the Ghost from Rebels made an appearance, we also have a MC70 from Rogue One and the Hammerhead corvette that appeared in both and is itself a reference to some of the old Star Wars KOTOR games.
Sadly based on the new trailers we're getting old Imperial 1 Star Destroyers for Team "probably Palpatine". They're a bit outdated but there seem to be a lot of them so that's not really an issue. We got a new TIE Fighter design at least. What does it look like again?
On second thought, maybe Disney should just stick with blatantly rehashing OT designs. Or maybe they could have used the TIE Defender from Rebels. Those Star Destroyers seem to have less hanger space than regular Imperial 1s because they each have some kind of giant cannon use for something so it makes sense that Palps would go a bit crazy with the more scaled back fighter compliment the Star Destroyers have to compensate. Either way I'm going to add the TIE Dagger to the ugly TIE _____ design hall of shame, which is basically all of the designs introduced since ROTJ except the TIE Droid and TIE Defender, Kylo's TIE Silencer from TLJ and the TIE Advanced ones the inquisitors use in Rebels which barely even qualify as a new design.