Not for the game I'm currently working on, I just thought of this idea because of the 4th version of Dungeons and Dragons. I just want to get your ideas. The combat system is as follows:
Every player/hero and opponent/monster does damage in one of three schools. To make it easy, let's call them melee, ranged and magic. Melee has advantage over magic, magic has advantage over ranged and ranged has advantage over melee (let's keep the argument of whether this makes sense lore-wise out).
On encountering an opponent, the player rolls two dice. If he has the advantage (e.g. player is melee and opponent is magic), the player chooses the highest roll.
The opponent has a threshold. If the threshold is exceeded the player attacks, if the roll and the threshold are equal none attack, and if the roll is lower the player receives damage.
I still need to think what would happen when equal combat schools meet. It would be possible to throw with 3 dice at all times, or just throw a single die in this case.
Like I said I have no intentions (yet) to use this myself, so if you feel inspired please feel free to use (parts of) this idea. I am curious to your guys' feedback :).
I like your suggestion to have an upgradable system. If you could increase the number of dice used, care must be taken what to do when one has a disadvantage, because obviously the chance for a low roll increases as well as the chance for a high one. Another option would be to change the die-type, as I do in my current game (e.g. from D4 to D6).
I also though 3 dice would make a lot of sense because then you have low, middle and high. However, the amount of dice in a system such as this has to be tweaked very well, because the chance for an extreme (high or low roll) increases with an increasing dice pool: imagine using 100D6, then every roll will get at least one 1 and one 6. Then you might as well not use dice and say "I have a disadvantage vs I have an advantage".
Therefore my initial thought is that players that encounter an opponent with the same combat school use a single die.