Complaints that a new faction will imbalance the game show so much faith in the designers... Starcraft has three factions and balances out just fine.
Shared units offers a lot of fixes we've all been wanting.
Want a Rebel Bomb Dropper. Oh look, the Firespray.
Want an Imperial Turret Ship. That Mercenary y-Wing that lost its R2 slot for a crew slot.
Etc.(Although please don't do this. The Doom-Y would be a way too funny to look at. But as a nuetral faction ship it could easily be limited to neutral crew only.