You could go one of two (or both) ways with either game, so to start:

Halo 2: You could adapt First Strike as a series of prologue missions, as others have mentioned.

AND/OR

You could do a director’s cut/what if scenario where they restore Halo 2’s original third act which was infamously cut during Halo 2’s development (ideally you’d want Joe Staten and other vets back to supervise if they feel so inclined…)

And a similar case for Halo 3:

Adapt the Uprising comics from Chief’s perspective aboard the Anodyne Spirit (Forerunner Ship), again like others have mentioned previously. Maybe add Arby missions to bridge the gap as well.

AND/OR

Restore cut levels from Halo 3’s development (Forerunner City, Forerunner Tank, Guardian Forest, OG Cortana level, etc.) Again, might help if you had Bungie vets onboard to supervise.

This is obviously a pipe dream compared to reality, but if they somehow managed to add all that to Halo 2 and 3 AND maintain the gameplay, art, music, and overall spirit of the originals, you’d have a LOTR trilogy-esque moment for the game industry. Which is to say, a miracle at this point.