How do you prevent the main character from being stuck as a grunt after saving the world many times in a long story?

[Meaty Clackers]

Not sure of a static solution but it would be interesting to play through an expansion where your character is granted a major domain and you play through the grunt quests as your upstart vassals then handle the major dungeon/raid threats as your MC

[TKVNC]

I'm not sure how to solve and properly do a reputation system, but I will say that Skyrim's is hilariously bad. One second you're the legendary Dragonborn, next, you're some shmuck who the guards and NPCs will openly disrespect

Morrowind does this already, and pretty well. Related factions also have appropriate splash gain, i.e House Redoran reputation also improves your reputation with the Temple, and the Buoyant Armigers.

Mount and Blade also does this quite well, and it's improved on by a few mods. Lords do respond to you station accordingly, and your reputation. Gekokujo and Viking Conquest also do a very good job with villages, and open world interactions.

As for scaling threats, Morrowind and Mount and Blade both do pretty well, Morrowind you remain an adventurer, but you gain support to make your mission possible, and with Mount and Blade you go from odd jobs, to gang leadership, to a minor vassal, to King.

Some mods, specifically Freelancer have you carry out soldiering, with promotions based on skill, and relationship with your commander - and you can be set missions too from time to time.

Really, it's lack of Dev thought most times.