Ok so, some time ago the decisions was made to remove reputation grinding from dungeons.
I think this was a mistake and was only done for time gating purposes.
So... My suggestion here would be to give us 5 ways to earn rep, but SOFT cap us at a certain point.
Allow us to do the following for rep
Delves,
Dungeons,
World Quests, - Might have to change how these work for it to be viable, maybe each time you complete one a new one appears.
Mob Grinding,
Raiding
Each day you can gain a maximum amount of reputation, lets say 5000 (probably much less but as an example). You can pick to represent a faction, you can change who you represent any time by visiting the quartermaster of the faction (or just bring back tabards for this).
Once you hit the reputation cap, you can keep grinding via your preferred method but all rep gains will be reduced by 95% meaning the most you're going to get is like five rep for a boss and maybe 1 for every couple of normal mobs and 2 or 3 for an elite pack (from like a dungeon).
This allows 3 things
You gain rep via the method of your own choosing
You can choose to grind it out or just do your daily
You passively gain rep just playing regardless of the activity