From trying the Insider build of the Halo Reach portion of the Halo:MCC I've come up with a few options I'd hope to see in a trainer. These are just potential options and do not reflect on anything that will be in the trainer (if a trainer is even possible on release).
Unlimited Health Unlimited Marine Health Unlimited Armour Infinite Ammo No Reload Instant Ability CD Unlimited Sprint Infinite Grenades Super Player Speed Easy Kills Low(er) Gravity No Overheating
I'm sure the other games will have slightly different options that people would want but cannot know until it releases on PC. If you have any suggestions of your own feel free to post below. I'm sure this thread can help the trainer makers see what options are desired and allow them to find out if they're possible once they get a hold of the game.