I know that cross-classing is very heavily preferred by most people due to flexibility and the combinations it has.
But given Fighter as an example, which gets a Discipline Bonus, and Bonuses to Their Helmet, Shield, and Armor AC for going pure fighter. I think it would be nifty personally if every class had a "Capstone" style ability. Sure they don't need to be competing with cross classing benefits, but it would be nice just to sort of have something for sticking to your main class and becoming the pinnacle of that class.
Some classes get benefits that do scale to max level, but other classes do not seem to really get much for sticking it out.
Just a thought and suggestion to mull over.