And, if a bottom line goal is to make the fighter "The Class You Start With", letting the player break the action economy a bit (within reasonable constraints) makes a lot of sense. I know I've played with people who just jumped out there and said "Wait, the giant is attacking him? OK, I'm going to interpose myself between them" and the rules would have the DM say "sorry, got to wait your turn."
To put my money where my mouth is...
[SECTION]Martial Reaction. Starting at 2nd level, once per round either on your turn or at the end of another creature's turn, you can adapt quickly to changing conditions in the heat of battle. You can Disarm, Grapple, Shove, equip a shield, change the grip on a weapon from one-handed to two-handed (or vice versa), or stow and draw a weapon for free.[/SECTION]
As a design note: You'll see the example I gave above about "interposing" oneself isn't possible. That's because I see subclasses building on Martial Reaction, just like the Thief subclass builds on the rogue's Cunning Action with Fast Hands.
Basically, it's an adaptation of a monster's Legendary Action for the fighter, restricting the options to non-damaging ones, restricting it to 1/round, but letting the fighter take them on his turn OR after another creature's turn.
