What kind of mechanic could catch the feeling of a warfare battle.

You could probably create a word-card system to accomplish something close to "unlimited possibilities." Have players draw cards with words like "jump" "feint" "behind" "around" "the enemy" "archers" etc. Each card has a number, representing its "power," and when a player plays them as a sentence, they can add up the numbers for additional dice, movement, forces, or some other benefit.

You can make this as granular as you like -- different piles for formations, verbs, nouns, whatever -- or very general, with one big pile. You can make the scoring very simple -- one kind of bonus, added together based on each card -- or more complicated -- different types of possible powers on each card, but only being able to score one of them per sentence; possibilities for unique combinations granting extra-special powers. You can also have some cards, when played, offer specific benefits, such as added forces, an automatic kill, or even drawing more cards!

Balancing would be hell, but if you could pull it off you'd get a really cool, really flexible element to add to any wargame. The mechanic you want to explore is "storytelling." I'd start by looking at "Once Upon a Time," a game that uses hands of word-cards to help players guide a mutually-told story to their unique ending. "The Adventures of Baron Munchausen" is also a good place to look, being a very loose role-playing system based on extraordinary tales of tall adventure and derring-do.

The obvious answer to me is: Play a role playing game

But that doesn't help :)

One thing that comes up in your examples of movie style combats is that individual characters are important to the outcomes of the battle. So having the players control and identify with heroes within the battle would be neat.

Another thing that comes to mind are interactions on the battle field in different areas. Like, we have to hold this gate so our reinforcement can rally and push in on the enemy. Or If we distract the enemy on this side this gives our calvalry time to ford the river on their flank.

one way to do this might be to have maneuver cards, that if one prerequisite is met, you unlock an ability for another unit. Like, "If your infantry scores a critical hit, you may instantly move a calvalry unit" or "If your magician is eaten Lemurs, you gain control of the dragon unit for one round"

Picking a scale is important too. You want it small enough that a single hero can be useful, but large enough that the greater battle is still important.

Adding subjectivity is a dangerous thing in a board game to me. Especially a war game. One thing that I like about war games is that things are cut and dry, everyone understands the rules, and the outcomes should be agreed on by everyone. Even a game like Warhammer has too much subjectivity for me, with the measuring, and figuring out which models are base to base etc. Give me a hex map any day.

And that is the point where I think that roleplaying games do so much better than board games, creativity in strange situations are the rule. The players agree that they have a mostly neutral arbiter of the outcomes, but if you want to 'break the rules' you can, just make a house rule on the fly. I suppose you could try to apply that same kind of rules on the fly logic to a board game, and that would be an interesting challenge!

Good luck! I would love to see the game you are imagining.