I do not recall where I framed some semblance of this idea but here goes nothing.
Apart from approaching the same missions in a different manner, do you think the following would help make the game a bit easier to revisit?
1. With each rank or set of missions (for example, 3-1 vs 3-2), or specific achievements of the highest order (Gold Survivor) NPCs within and out of missions regard you differently, giving way to new key points about the overall plot within the game at least?
For example,
= begin a mission prematurely by invitation through a player of a higher rank and you don't get the regular dialog or you may get talked down to like "Who's the shrimp?" or "Are you sure you're ready for this soldier, recruit?"
= begin the same mission when it is of your ranking to get the same dialog as you'd get now with missions
= begin the same mission when your rank is any measure above the mission's ranking; certain NPCs will divulge more with the more you progress through the game, be it idle chatter or plot exposition.
= complete a shadow lair level (specific or general) and maybe there is praise or something not as kind in their tone of regard.
The only way to get the regular dialog when the mission ranking is below your ranking is to play on Normal Difficulty.
2. Once the regular campaign has been beaten to grant you the title of vanguard, you are granted access to a mode more difficult than the Elite Difficulty where the only place you can get consumables (vials and health caps if I have that right) are from NPCs in the spaces between hubs AND the crossroads, and where emergency revival is not available.
You are technically saying in short words that you want more interaction between the NPC's and the player itself such as saying different things depending on the difficulty in which you played the mission?
If so the 1+ from me and add this to the suggestion category :3
The only issue i see with that is that it will take a huge amount of time for the team to script all the things which you have described :/ so i might actually happen but no way near the future