So my idea for this map is that basically you are just trying to push your way throughout the map and reconquer the world from the darkness. I am incorporating an ammo system, with ammo packs along the way with random amounts of ammo. You have a marine, firebat, ghost, and medic (4 players) the medic eventually changes to a kerrigan (but still heals). Now I'm trying to figure out ways to make it challenging to push through while some of you may run out of ammo, others may not. But if you wait to long you will be out of ammo, and have no way to push through. Any input, suggestions, comments, compliments, complaints? Also anyone that wants to help me out with terrain is more then welcome

Its a good start but those seem to be common gameplay mechanics among the genre. You might want to try being more innovative/creative or at least have a great presentation if you want to stand out.

You could have it so that ammo crates respawn after some time via burrowed units, but I think you're just trying to make a generic RPG with an ammo system. Another thing you can have to spice it up is towns with ammunition dealers, and other necessities. Of course, you need a currency system, though. And also, try to beef up the storyline, the whole "save the world from darkness" thing is too generic.

Along with ammo, Health packs and gernades go great for the whole infantry feel. Incorporating these would make it cooler, maybe a+left click to shoot or something.

I have not been able to find a game where each person has a special role, and timing is crucial. If such a game exists, then I'd love to know!
Well, the health packs would ruin the point of the medic. Grenades would be cool though, I'll have to try a system for that.

Hmmmm not exactly sure what you are asking for but...

I think it would be interesting if you made it so that each unit/player (marine, firebat, ghost, and medic) has its own set of problems/challenges and that they are all dependent on each others existences and well being. Perhaps along the journey there will be "towns" or "bars" or something where they only allow women to enter (hence only the medic) and there will be plenty of ammo for the medic to restock on. Maybe there's really "hot" places like a cave of fire or something where only a firebat can withstand its heat.

Not sure about most people but I am a big fan of puzzles. Perhaps you could incorporate some puzzles or clues into the map where solving such things is a requirement. Like, perhaps along the way you will enter a casino where you gamble using your ammo packs as currency or something. Give the feeling that your ammo packs are precious and very valuable.

I don't know, just some ideas off the top of my head...