Here's the Counterspell portion. It's an opposed check (d20 + spell attack modifier + level of the spell/counterspell. The idea is to allow the sort of magical battles a la Harry Potter or other movies/books/etc.:More or less ditto, the specifics differ but the idea is the same - with one notable exception: a counterspell never triggers a WMS (I see that as a built-in part of how counterspell would work) nor does being interrupted by Dispel Magic.
I don't bother with the modifiers, but if your spell is interrupted etc. then it's lost (and thus won't resolve as normal)...at which point we go to:
Wild Magic Surge table:
(with a tiny bit of rounding I could use d20 for the base table but as I'll need d% for the effects table anyway I use them here too)
01-50 spell fizzles, maybe get a soft 'pop' or a small puff of smoke or a brief glow
51-70 full reversal if possible (e.g. cure wounds becomes cause wounds, darkness becomes light, etc.) or see below
71-85 minor surge, table 1
86-94 significant surge, table 2
95-00 major surge, table 3
if reversal not possible then 51-62 = minor surge, 63-68 = significant surge, 69-70 = major surge.
Tables 1 2 and 3 referred to are the actual effects, ranging from silly to deadly to extremely beneficial and sometimes combinations of all three. Each table has about 97 entries, with the remaining gaps saying something like "just make spit up". As their names would indicate, the effects increase in severity as the table numbers go up. Each entry also indicates who might be affected out of caster, target, or what I unflatteringly refer to as SPIN; which stands for Some Poor Idiot Nearby.
So, at most I usually only have to do 2 dice rolls: one to see if there's a WMS and sometimes a second to see what the surge does. Sometimes subsequent rolls are required (saves, damage, etc.), case by case.
The WMS table also sometimes comes into play if a magic item gets broken; but the more common result there is a simple 'boom' as the magic contained in the item gets released all at once in a way its designers never intended.
Lan-"I've also introduced jumped-up Wands of Wonder called Wands of Wild Magic, which also use these tables except without the fizzle chance"-efan
The caster of the original spell checks on the Counterspell table:
-5 or less The counterspell succeeds and the original spell is canceled (and the spell slot is lost).
-4 Roll on the Wild Magic Surge table.
-3 to +3 Contest continues. You must maintain concentration, cannot take other actions or reactions, and must make another opposed check next round.
+4 Roll on the Wild Magic Surge table.
+5 or more The counterspell fails, and the original spell is cast as normal.
The caster of the original spell rolls on the Wild Magic Surge table if appropriate, and applies the +/-4 modifier to the roll. If the result on the Wild Magic Surge table is a backfire, it affects whichever caster has the upper hand in the contest.