|Via LevinLight Wiki|
You can get more extreme with this, in terms of abandoning the spell slot system, but I want to work this out sticking as closely to the classic fire-and-forget casting system as possible. Another insight: "no result" is boring and frustrating for spellcasters. So I'm just excluding it as a possibility.
Whenever a magic-user is going to cast a spell, before the player has stated the targeting and area of the spell, roll d20.
The spell goes wild on a 1 if the caster can use spells of a higher level than the one being cast, or a 1-2 if the spell is of the caster's highest level. Add 1 to this wild range for every 2 points the caster's Wisdom is below 9, and subtract 1 if the caster's Wisdom is 13 or above.
The spell goes critical on a 20, a 19-20 if the caster's Wisdom is below 9, and 18-20 if it is below 5. There is no chance to go critical if Wisdom is 13 or above. Yes, magic-users may want to lose Wisdom to become more powerful and random, which is why some spend so much time delving into secrets of forbidden knowledge (insert sanity loss=wisdom loss system here).
Wild Spell: Roll at random on the list of magic-user spells for that level. The magic-user casts that spell instead, whether or not he or she knows it, but may target it at will. If you rolled by chance the spell the magic-user was trying to cast in the first place, the spell escalates; go to the next highest level, the magic-user picks a spell that will not be cast, and roll on that spell level table to find out the spell that is cast. If the spell picked is rolled, escalate to the next highest level, and so on.
A creative DM may have some of the spell's effect or style influenced by the spell originally cast. For example, a magic-user trying to cast Web who casts ESP instead may find that cobwebs appear on the walls, that shout out and echo the thoughts of the closest sentient being.
Critical Spell: Roll 2d6 and take the lower:
1: The spell is cast with double range, double duration, + 1 to each damage die, and -2 to saving throw.
2: The spell, once cast, is replaced in memory by a random spell of the same level.
3: The spell is cast with double effect (area or damage, for example) or with -4 to saving throw.
4: The spell is cast without using it up in memory.
5: The spell has its usual effect plus the effect of a spell the caster chooses of the same level or 1 level higher.
6: Roll twice, and 6 has no effect.
I think this system has a good chance to walk the line between player boredom and total player screwage. Instead of "you fail" it's more like "hey, make the most of Rope Trick right now."