Then a simple d20 mechanic would filter in - perhaps opposed rolls.
Modifiers could be spell level and ability modifier and perhaps 1 for every 2 levels of caster (whatever takes your fancy).
- A higher result by the original caster would mean his spell goes through.
- A higher result by the counterspeller would mean the spell was negated, and/or perhaps his spell went through instead.
A pass by 10 or more by either result could lead to fantastical events (DM adlib)
I got it!
Would you all agree that a 9th level spell is harder to cast than a 1st level spell? If you concede that, whatever counterspelling rules need to make the DCs decrease by 1 for each spell level.
So how about this
Counterspell DC = 10 + Level of enemy wizard - Spell level - your int mod
With the caveat that you need to consume a spell of an equal level. I.e. you need to waste one of YOUR 9th level spells' magic in the process. We could even add a feat allowing you to spend a lower level spell instead, increasing the DC by 1 or 2 per spell level. I.e. if you are both 20th level, the DC is 21, less, say, 5 for your int mod, so 16. If you want to spend an 8th level spell to counter his/her Time Stop or Wish, you need to beat 18. Disintegrate? 20.
Another feat could improve that further, to reduce the DC cost to 1:1.
This would have the nice side effect to coach wizards in duels to "waste" lower level spells first since they would be harder to counterspell. Or if you are dueling a higher level enemy, and he's casting a 9th level spell, you simply CANNOT counterspell it unless you can also cast 9th level spells as well, or have taken those feats to counter higher spell levels than you can currently cast.
Either that or make higher level spells harder to counterspell, and take both your levels into account in the DCs
DC = 15 - your wizard level + theirs + spell level
So a level 9 wizard vs a level 1 wizard casting a 1st level spell, would have a DC of 8 to counterspell it. If you're equal level, the DC is just 15 + spell level (plus consuming a spell of an equal spell level of your own)
I HATE memorizing Dispel Magic. It's almost always needed more than once when it is, and forget about readying an action. Those 3.5 rules SUCK. Immediate reaction is goooood. I need to re-read the playtest rules.