One day i will run out of things to suggest, and the world shall celebrate, anyway.
Marine Classes
All stats apply to marine combat only
Standard Marine:
Description: The bog standard marine, equipped with the standard blaster rifle and minimal armour for lowered cost, maybe if these guy's actually saw some real combat instead of just VR they may be of some use.
Cost: 1 credit
Attack: 1
Defence: 0
HP: 1
Veteran Marine:
Description: Looks like your marines actually managed to survive some real combat, they now know how to improve there own armour and weaponry from salvage, still ain't much use though.
Cost: None, they are standard marines who have survived several fights (maybe a marine exp like the ship exp?)
Attack: 3
Defence: 2
HP: 3
Heavy Marine:
Description: These guys are the standard marines, but supporting a plasma launcher as well, let em through the hatch and watch the fireworks, still there armour sucks, maybe it's time to find a new person to sell you marines?
Cost: 5
Attack: 5
Defence: 1
HP: 1
Veteran Heavy:
Description: Looks like his Plasma launcher kept him alive for long enough for him to get some real combat experience, he knows how to use his weapon and has salvaged some shields to improve his chances for survival.
Cost: None, hes just a heavy with some battle's under that launcher
Attack: 8
Defence: 3
HP: 4
Guard Marines:
Description: These marines are mainly for guard duty, they can hold there own in a fight, but don't expect them to fight the enemy on there ships if you don't give them some support, these guys have the standard blaster, and a portable shield, but it takes time to set up best used for defence.
Cost: 5
Attack: 1
Defence: 4
HP: 2
Veteran Guards:
Description: Looks like some guards actually made it, you should be proud, they've salvaged larger shield's for themselves and have found a nice shiny blaster to compliment it, i still wouldn't let em in on there own.
Cost: None, they are Guard marines with Experience.
Attack: 3
Defence: 6
HP: 4
These are just some example's, there is also some research suggestion's i have to go with them, but I'm no good a research cost's so they aren't included:
Marine Training:
Description: let your marines shoot at each other with blanks, hell, they might actually learn something.
Bonus: Marines Gain experience faster.
Gun Refinement:
Description: Get your marines to work harder on weapon's research, and make sure they get that shiny new blaster they always wanted.
Bonus: Increased attack for all marines Yippee!
New Armour:
Description: Treat your marines to that new piece of anti blaster armour they've been staring at in the shop window, whats the worst that could happen?
Bonus: Your marines seem to live longer, guess that means more pension's to pay out.
Better shielding:
Description: get your marines the latest shield modules, and watch as your opponents blaster's shoots melt away, warning side effects may include, Heroism, belief of immortality, and more marines running into dangerous situations
Bonus: Increased marine defence, yet more pensions to pay

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Power Core Refinement:
Description: your scientists seem to think that better powercores on marines backs might actually make them live longer, well even if it goes horribly wrong it'll just mean less pay cheques, so its a win win situation.
Bonus: increases all marine stats, and maybe how long they live.
Silence444 suggested:
Shield overloader:
Description: By feeding our own excess power into our opponents shield matrix we are able to shut of there shields, with a nice explosion to go with it.
bonus: Decrease enemy marines shields.