I've been giving some thought to action economy and realism in an abstract sense lately. To that end, I've started poking around more seriously with White Wolf's Exalted initiative scores and the impacts they would have applied to something akin to D&D.
Traditional D&D: Roll 1d20, add Initiative Modifier, sort all combatants from lowest to highest. Highest goes first.
This kind of system is an opposed check with a pretty large swing. Let's try this one...
Highest Initiative - Character Initiative = Delay. Combat starts on initiative count 0 (meaning Highest Init - Itself) and goes up from there.
Why the reversal of an already relatively simple system? To allow for more modular use of actions, and removal of "turns" as an abstract concept. Now instead of a sheet for initiative that loops around, you have a line that continues forward indefinitely. The benefits of this aren't obvious at first, so here are some examples:
Entering an already active combat? Your initiative is the current initiative + your delay (Remember, the ONLY number that needs to be kept from the start is the initiative score of the HIGHEST player, who set the pace for the entire encounter)
Want to split up your actions? If the system attaches a "delay" cost to every action, then the actions are split naturally. If it doesn't (like D&D), then you can simply take your first action and that is where your "turn" starts. If you don't use up the rest of the actions before that "turn" comes up, then they are lost. It's like a built in Ready Action.
That one was probably a little confusing...I'll convert a normal game of D&D to the new system to serve as an example:
Bob has +4 initiative.
Jim has +8
Greg has -4
All of them subtract their initiative scores from the highest score.
Jim goes first, setting the pace at his 0 Initiative. He choses to perform only a move action.
(Jim is now "readying" his standard and swift action.)
Bob goes second at an initiative count of 4. He decides to perform a 1 round spell. This means he can't go again until all other members of battle are behind him in initiative.
Greg would be going third at a count of 12, but Jim decides to use that standard action he was saving to disrupt Bob's spellcasting.
Seeing that Bob and Jim are at odds, Greg decides not to act at all. Aware of the combat, he's effectively readying all of his actions and moving his initiative where he wants.
As no other characters are choosing to act, Jim is now up to act again.
As shown here, this system favors fast characters who keep their options open.
Let's add a new condition to counterbalance it a little...
Slowed: Whenever you perform an action, Your Initiative becomes the tick you performed the action.
(This would move Jim's initiative to the tick he used his standard action to interrupt Bob)