I've been playing for quite sometime, and would like to have the option of doing some of the old mission fights again, and have come up with a nice way to allow it.
For the nation missions that have a reward, I would suggest having to spend Seals/Crests to get an Orb, or maybe even use some of the Garrison trigger items like the Seven-Knot Quipu to gain access. For increased rewards accept a level cap if the fight originally had one such as the Rank 2 Mission 3 Dragon fight's level 25 cap, and also get a treasure coffer as well, and for fights that had no level cap, come up with an appropriate setting to allow a treasure coffer to drop at the end, such as a level 75 cap for the rank 9 mission 2 fights.
If you don't accept the level cap you get 20% of the original rewards or something along those lines
For the other types of missions like Chains of Promathia and Treasures of Aht Urghan, similar approaches can be made, still use Seals/Crests, or an item related to the mission, such as Animas for the Promyvion fights. Use the original level caps to be eligible for the coffer of course.
To also be eligible for the Coffer version, you must have completed that group of missions ie: Rank 10 for the nation missions