Level Based AC bonus
This is really my big change, no one has one and its sorely needed especially for low magic games
I use this rule in all my current D&D games and it works at least at low levels in both old and new. Basically each round declare a stance and take the following changes.
Neutral (no penalties or bonuses) this is the default stance
Reckless -4 T0 AC +2 to Hit
Aggressive -2 to AC +1 to Hit
Defensive +2 to AC -4 to Hit
Cautious +1 to AC -2 to Hit
Full Defense +4 to AC No Attacks
4 Save system , 3 of them are ala 3e (the older system is too cumbersome for my taste) and an additional perception save. I will also use passives saves ala 4e or the SRD variant.
Heroic feats/Called Shots
I want a good system to handle this and everyone should be able to do them with the caveat that Fighters should be generally better .Rogues and Rangers might also be good at a few tricks I am not sure.
Instead of a critical hit, a roll of a natural 20 would result in some kind of cool extra effect based on the weapon or monster.