I'd rather see vehicles hidden in specific levels. For instance an asteroid in the corner of a space map with a door needing several rounds of explosives to open, that would spawn a pirate fightercraft in an abandoned docking bay.
That kind of stuff adds flavour to levels.
The unlockable ships is still a good idea though. I'd be interested to see unlockable ships based on pilot stats, though. For instance if you're very accurate or have a great kill/death ratio with the TIE Bomber, you eventually earn a TIE Bomber with upgraded shielding, maybe some more armorment, and distinctive squadron markings.
Distinguishable squadron markings for unlocked ships would kick pie actually. To be a Rebel fighter pilot and see a TIE with 181st-style marking would surely strike fear into you and you know you're after a superior pilot.
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