One rule I like is based on the ticks system from Scion.
Basically, it determines Extra Actions based on your Initiative, not your Dexterity. Sure, Initiative is based partially on Dexterity, but it's a lot more fluctuating because it also involves a die roll.
Basically, you get 1 action every 5 points of Initiative, until you hit 0. Once everyone hits 0, you roll Initiative again (and since this is a system that uses Initiative every round, that should be fine).
So let's say everyone rolls for Initiative and X gets 17, Y gets 8, and Z gets 9.
X acts first, at 17. then she acts again, at 12 (because it's still higher than the next highest Init).
Then Z acts, at 9.
Then Y at 8.
Then X again at 7 (12 - 5).
Then Z at 4.
Then Y at 3.
Then X at 2.
At this point, if you subtract 5 from anyone, they hit 0 (or negative numbers) and can't act. X's last action was the last action of the round. Roll Initiative again.
Etc, etc.
I also typically allow people to defend reflexively, but if you want them to have to spend actions defending, just allow them to do it sacrificing their next one (if they have one).
Keep in mind all this still comes with the cumulative -2 penalty per action until the round refreshes.
Another good alternative rule is that you take a penalty equal to the amount of actions you take, which is fixed.
X wants to take 3 actions this turn. Her player states as much, and now every action is penalized by -3, including the first. Spreading thin affects everything you're doing--you need to keep a little in reserve at the beginning of your assault for the rest of the actions.
This makes it so that people who do less during a round are more effective, while people who try to do a lot might just get sucky results across the board.
This last one really necessitates reflexive defense.