I'd most likely try and become a Spartan IV. If not, then just being a marine or ODST would be enough for me.
As for being something that isn't human, maybe an AI or a Sangheili warrior, idk.
If I wasn't in any military faction, you'd probs find me exploring the vast reaches (no puns here) of the universe and maybe even exploring a halo installation if ONI lets me.
Speaking of ONI, I'd maybe do the usual stuff spooks do if I was in ONI. YK, the spying, sometimes backstabbing, and generally just tending to whatever ONI section three project I've cooked up.
However, being a monitor sounds interesting, so that might be something to consider. Being in control of an entire installation built by the forerunners would be so cool. Maintenance however, could be a nightmare. The sentinels would be awfully helpful in the event of an outbreak of flood.
If I ever got infected by the flood I HOPE TO GOD I wouldn't have to go through what PVT. Jenkins went through. The novelisation of CE portrayed the truly nightmarish properties of the flood in insanely creepy and nightmarish ways.
As for being turned into a promethean, It would be very annoying having to listen to the Warden Eternal's ramblings and constant moaning.
If I was a forerunner, then rip me. Unless I was shoved in a cryptum, in that case I'd most likely learn my lesson or end up nearly killing all of humanity...
Out of all these options idk what I want to do if I was in the halo universe. But you definitely wouldn't find me as part of some fanatical covenant or the insurrection. And I'd definitely NEVER be a jiralhanae. If I had no choice in any of these things and I just got shoved into a random role, I'd probs try and make the most out of it.
If none of this is possible, then just living on reach is fine.