Okay, one thing to realize is that a character's Defense is MORE than just a static number. That static number is simply the base from which the defender starts off...
First off, there is
Fighting Defensively, which allows a character to swap their offensive capabilities (i.e. their attack Bonus) for increases to DEF at a rate of 2 to 1 -- this is actually more of a stance/style of fighting that basically makes the defender harder to hit.. Then there is Dodging (i.e. getting out of the way of an attack) and Basic Block & Parrying (i.e. attempts to stop the incoming attack).
You have to consider these before you can start thinking about making Defense an opposed roll...
Just off the top of my head, for opposed rolls for combat, I would possibly do something along the following lines.....
Opposed DEF Keep all of the standard modifiers to DEF, and add in a base of 5. (i.e. essentially DEF is equal to standard DEF - 10). And then let them roll their DEF each round. This allows for some really crappy rolls, and some really good rolls as well.
On the bad side, this means EXTRA rolls every round, and will slow things down a bit during combat (especially for the GM), plus players will get very very upset when they themselves roll badly..
Option -- you might also consider having DEF be affected by all melee skills (i.e. +1 DEF for every 5 ranks of ALL melee combats skills totaled), but if you did this, I would remove the +5 base I suggested above
You would almost have to go with something like the above, unless you want to completely rework how combat works (to change Fighting Defensively, Blocks, & Dodges)
