Now this topic has been discussed in other threads before, but to my knowledge* there hasn't been any threads dedicated to the idea. The list of current ships that are used by both factions in Canon are:
1. Nebulon-B Escort Frigates
2. CR90 Corvettes
3. Quasar Fire Cruiser-Carriers
4. Imperial Star Destroyers**
Now assuming that these ships have their stats and cost unchanged, would any of these break game balance? Would a swarm of Vader CR90s or Rieekan using Quasars be too powerful? Do you even want dual faction ships, and how should FFG implement the idea?***
*Roughly equivalent to Jon Snow's.
**Despite the numerous instances in Canon/Legends of Rebel/New Republic ISDs, I think allowing that in-game would steal too much of the Imperial faction's identity.
*** I am not a FFG employee, although I wish I was.