I'd like it if there were different types of vehicles like... More air vehicles and water vehicles. Some ment for just transport and others that are fairly stationary but fire far and with a mortar or something like the scarab.
Not only that but I'd like a wider array of levels, my favorite are city but there are so many that take place on Halo, High Charity and all that stuff. Anyway, I'd like it if there were levels in forests and maybe if there were water vehicles, missions on the water.

Water, snow, desert, mountain missions would all be interesting.

Desert Level
"Come on, Chief, let me borrow your helmet..."
"No, I like the air conditioning."

I want the Scarab and i want to be able to use it in the campaign not in multiplayer though that would be cheap

Scarab in Multiplayer wouldn't be that cheap if there were lots of high ledges above it that you could shoot and board from without being in sight of the gun. It would probably only be for one level specially made to have it, but I think it would be fun.

Desert Level
"Come on, Chief, let me borrow your helmet..."
"No, I like the air conditioning."
:D I can just imagine the scene before a desert mission. :D