Basically actions could dynamically reduce your initiative for the following round.
For example:
Moving during a turn cost 5 initiative
Casting a spell could cost spell level squared (maybe modified by intelligence)
Attacking costs the rating of the weapon (daggers would be low, two handed weapons would be high)
Special actions could also have a cost attached to them
Leaving a ZoC during combat could also carry a bonus 5 point in addition to the 5 for moving.
So if you have you have a character who has an initiative of 30 who leaves a ZoC to attack another enemy with a sword (initiative rating 8) he would then only have an initiative of 30-5(move)-5(leave ZoC)-8(attack with sword)=12 to decide when he next gets to move.
This would then make a defensive line much more viable, since when the enemy does attack you will have a massive initiative advantage on the following rounds to counter. Also it would reduce kiting etc.
Unfortunately this would require a re-write of a lot of the combat mechanics though.