Followers and stronghold...? well, I'll circle back to that...
...but, enhancements for the Fighter. Sure, especially that might help the champion without making it too much more complicated.
- Extra (Opportunity) Attacks - when you use the Attack Action on your next turn, the first Opportunity Attack you make, before you take your Action on your next turn, doesn't consume your Reaction. If you have two or tree extra attacks, the first two or three OA's don't consume your Reaction.
- Extraordinary Athlete adds it's bonus to Proficient checks, as well as non-proficient.
- Indomitable grants an automatic save instead of a re-roll.
- More & better, level-gated, Maneuvers for the BM.
- Single-class Fighters gain proficient CON saves plus choice of STR & WIS or DEX & CHA
- ...and something out-of-combat-useful for the other two pillars...
...which brings us back to followers & strongholds.
It'd be quite the undertaking and change the scope of the game, but one could expand the social pillar to include politicking & diplomacy and the exploration pillar to include exploration & colonization. And the combat pillar to a mass battle system, of course.
In the classic game, each class could get into doing things like that - the Fighter could become a feudal Lord, the Wizard have his foreboding tower, the Cleric a temple, the Thief a Guild, etc - so bringing them back is one obvious possibility. But having the party, as a whole, build up a community might also be an interesting way to develop things.