I have a PC who wants to jump onto an enemy in a combat situation. The player wants him to land on the (humanoid) enemy, sword drawn, and use the enemy to break his fall while plunging shortswords into it. I thought of using a straight attack roll, because it was only a 12 foot drop from the ledge to the top of the enemy. But I would imagine a lot would depend on the distance, and also whether the enemy might suspect such an attack. Also, I would expect the PC would take damage on a miss, but how about on success?
In previous games, I have adjudicated this by having the attacker take falling damage, and the target take extra damage as if from a falling object. It made sense in third edition because it adhered to existing rules. However...
The Rule of Cool states: The limit of the Willing Suspension of Disbelief for a given element is directly proportional to the element's awesomeness. In corollary, you should encourage awesome and interesting actions like this.
Now, I'd have it as follows: If the PC hits, they deal 1d6 extra damage for every ten feet they fell. The target breaks their fall, so they take reduced or no fall damage (say, ignore the first 1d6, or 2d6, or half of the dice rounded down so that falling 12 feet suffers no damage, on a hit). If they miss, the attacker takes the normal fall damage.
This makes for an interesting choice, because you're taking an additional risk for additional reward.
With two-weapons, for awesomeness, you might rule that as long as one hits, it deals the extra damage, though if both hit, the extra damage should only be dealt once.
Should you fall onto a Huge or Gargantuan creature, consult the Dungeon Master's Guide rules on climbing onto a creature (p. 271) and play an appropriate soundtrack.