Uhm...I have a possible idea for a new character class or two...
'Security Personell'
Basically the police and home defence forces already stationed in the complex. Probably starting off equipped with the .45 pistol and a shotgun instead of the marine armament? uhm, experience bonuses would probably be on shotgun-type weapons, and armor. Maybe if you implement the auto-turrets, If you're this class, you dont need technician skill to re-activate them, and they'll be forced safe. You could still encounter 'rogue' turrets previously activated.
'Synthetic'
More of an all-round class than the marine. except needs more experience to advance levels in anything (maybe 10-15%?), countered by the fact that the aliens would pay less attention to them, especially 'facehugger' types, If their implemented. If possible...Give them a kind of 'ressurection' after death, maybe at 25% health, with 30% 'permahealth'?
'marine commander'
bonuses to perception(10), technical(10), and rifles(5) and sidearms(5). This one would probably hinge on you implementing the motion detector from another thread, but have the commander equipped with one to counter the experience cost.
Just a few basic ideas...I havent played the game much, so my idea of the experience costs is probably wayyyyy off.