I vote for new Firaxis X-Com's Time Units. It really streamlines the whole gameplay without sacrifacing any decisions (actually, this simplification allows more decisions). Instead of counting all the time how many APs you have left, you just decide what you want to achieve.
Pardon my gushing, but: They're amazing, right? In X:EU, you only get two actions per turn, but you still have essentially the full flexibility of the original UFO:EU (except, of course, you can't shoot before moving). Technically, it's just AP with the "attacking ends a unit's turn" restriction, but it's interesting because there are just enough AP to force you to carefully budget them while still having so few that you can instantly know whether you can or can't do something, without having to look it up. When combined with the movement radius your troops have, the effect is to move the focus from when things happen to where.
It's like how the original Command & Conquer was able to have a relatively deep strategy game with fast-paced, engaging action, without making the player internalize a dozen spreadsheets and a whole keyboard of macros, just by giving the player a clean sidebar and a single, contextual mouse button. Or how Deus Ex was able to turn a stealth-focused FPS into a deeply customisable, pseudo-RPG experience with several skills, measured on a scale of 1-4, a handful of binary augmentation choices, and a limited inventory.
On a sidenote, turns can be resources/advantages as well, in some games characters have speed or initiative that allows them to take their turn, so higher speed allows more turns. As an extention to this, to add in extra strong abilities i would have a character "prepare" one turn(possibly taking his next turn earlier) so he can use the stronger ability once.
You mean like Final Fantasy Tactics (and probably a myriad of games I'm not familiar with). That is an oversight on my part, but in my defence, I hadn't actually played that game until a few days after posting.
On another side-note:most turn-based games use mana to perform most actions instead of action-points(1 action/turn, X mana the whole battle or more if you defend often or something)
I'm not sure I've heard of this one, before... What's an example? Also, what happens if both sides run out of mana without destroying the other? Does it double as morale, and the side/unit that runs out retreats/surrenders? Or perhaps they fall, lifeless, like robots without any battery power?