So everyone knows how in Halo you always are a Spartan with the exception of being the elites... But why not continue with that course, but instead maybe be a Marine or ODST? Say there are two buildings or objectives and a Spartan is sent to clear out 1, and a group of Marines or ODST are sent to the other. You play as the Spartan reach a secondary (not primary objective) and then it brings you over to the ODST or Marine. You then are only as strong as an ODST or Marine health wise and physically, so beat downs aren't as effective. So its is more of a challenge (and in some cases failure is imminent and may be part of the story). After reaching the objective or a secondary objective, or even having a story planned death, switch back to Spartan to reach the final objective, and let a story unfold (like hearing the ODST or Marines didn't make it, and you must go in to reach the objective while the covenant are also moving in). I think switching off would extend the campaign, and let everyone see an extended story behind what actually happened.
Now if switching between characters there should be hone rule, or one law to keep it real. The characters you play as never cross paths. Or should they?
How cool would it be to play as a Marine, pinned down, heavy fire, you get shot, nearing death. Then, just as you are about to die, you watch a Spartan run into the middle of the fight. The moment this cut scene happens via the marines perspective, you switch to playing as a Spartan! There are many many scenarios that could make the game brilliant. But here is a poll.
With the information above, would you want to see the characters you play as cross paths? Granted you don't always play as the same Marine, ODST, but always the same Spartan. So you may catch a glimpse of the Spartan, never see him, or he may swoop in and save you (and you play as him). Please vote!!