I don't think we should have any system like 'fatigue' or 'mana' points because we've already tried these things over a period of almost 30 years and they never worked.
We need to take an existing element of the game and quantify it. That element is an 'action economy'.
Now to make spells work in that action economy, the spells need to be near equally deadly and balanced. That means if 1 target takes 12 damage that 2 targets in a burst should take 6 damage each. Spells that cause helpless conditions should not be burst. Weapons should do 1[w] damage.
When we limit the damage of the power then we can make them all at-will and allow multiple actions per turn based on level.
When play-testing this, it turns out that multiple reactions outside of your turn does slow down the game a lot. But multiple actions per turn doesn't.
This is what I'm currently thinking.
Martial characters get 1 action or 1 reaction per turn with an additional action every 5 levels. At 5th level, 1 reaction and 1 action or 2 actions. They don't however get more than 2 attacks with their main weapon. They must use a shield, a stomp or an off-hand weapon for the remaining attacks.
Casters who use enchanted weapons in melee or ranged attacks they get the same action dynamic as long as it is the same power -- for example 3 Magic Missiles at the same target or multiple targets. But when they use a helpless-target power it costs them all their remaining standard actions.
