I think that designing a combat system to be elegant and expandable is possibly the hardest thing in this game. Anybody can throw a thousand rules out there, and patch together a system (I'm looking at you, Palladium), but it's hard as hell to have as little as possible.
In my case, the game I'm designing has two sentences on how to roll attack and damage. The attacking is easy: Roll underneath your Agility + Attack - your target's Defense on a 2d6. Minimal mental math, right? All you need to do is compare a couple of one-digit values to a pre-determined score.
But the damage formula is giving me a hissy fit. Ideally, it'd involve rolling your Might + Weapon Damage versus their Toughness + Armor Bonus. But the game uses a roll-under system, similar to the one used in old editions of D&D, and in Labyrinth Lord. Roll underneath your attribute for a success, roll over and fail.
In addition, I'm trying to keep the dice rolls down. What I'd had before is comparing the defender's Toughness + Armor Bonus to the attacker's Weapon Damage + Might, and that'd be the modifier on the Toughness roll. But as anybody can see, that's a relatively complex formula to do at the table, and not nearly as elegant as the attacking roll. But is there any solution?
Might should be an adjusting factor in your damage, just as Toughness should be a factor in whether or not you take damage. Ditto with Weapon Damage and Armor Bonuses. But I'm not entirely sure how to express that.
Perhaps roll the Weapon + Might into a Damage score, and Toughness + Armor Bonus into a Resistance roll, and then compare those to each other, like in the attack roll? And then use that result as the bonus or penalty to the defender's Toughness save?
It could certainly work, now that I think on it, especially since the point is that if you get hit, you're probably going to get hurt. A failed Toughness save can result in one or more damage, depending on how poorly one rolls. Getting hit by a sword is sincerely unpleasant, and you'd be lucky not to die from some of the larger weapons people bring with them.
But is my solution elegant enough?