<blockquote>Interesting idea - so, each time a combatant finished performing his action, all combatants' turn value would increase (and the combatant who finished will be reset to 0), right?
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Yes and every turn whoever has the highest turn value wins the turn! - but I think instead of resetting the combatant's turn value after it performs their action to 0 simply add 0. Other wise in a 1v1 fight it would still be every other turn regardless of a combatant's speed. Also in the event that two or more combatants have the same turn value, the one with the higher specified stat should win.
Icing on the cake would be if a number counter hovering over each combatant displayed the countdown until their next turn. Then things like delay attacks and haste effects could greatly be used to make battles very tactical.