I thought an achievement system might be cool in SC2. Achievements reward you more money. You can use money to buy ammo and hints, but money also serves as your score. I wish I could be able to create a high score table, dammit T_T.
Anyway, I think the achievements will be:
Got it done (+$10,000): Beat the game.
No one spared (+$5,000): Kill everybody available in the game. (Killing civilians don't count.)
No help needed (+$3,000): Beat the game without buying any hints.
Soft side (+$2,000): Help him up.
No mercy (+$2,000): Stomp him off to his death.
Flawless (+$5,000): Don't fail ANY missions.
One rich guy (+$5,000): Reach up to at least $50,000.
Accurate hitman (+$4,000): Fire no more than 30 bullets.

There is a "him" mentioned in the achievements. You will find out who he is in the game.

It's impossible to unlock all the achievements. At least that's the way I'm programming the game right now, but it's not my choice, I don't know how to fix it. The reason why you can't unlock all of them is because these achievements are unlocked according to your stats (like money, people killed, etc.), and when you start a new game, you start with new stats, and therefore, some achievements become locked again. For example, if you unlocked the "One rich guy" achievement, when you start a new game, your amount of money will be back to $500 (the amount of money you get when starting a new game), so the achievement will be locked again. I'll look for a way to fix this.

If you have any more ideas, PLEASE feel free to comment. I need -and respect- your opinions.