Most current spells either do not have "effect ladders" when cast with more WP, or they max out at ~3 WP. (The exceptions are exponential functions that need to be fixed anyway, let's not get into that.) This means the mishap ladder for the number of rolled 1's isn't steep enough. In most situations the chance of rolling three 1's is going to be just 1 in 6*6*6 = 1/216 because the spellbinder has no real reason to use more than 3 dice.
Basically I'd like the effect of the mishap to be related to the number of dice rolled, not the number of 1's rolled. Something like this:
* You roll at least one 1: Mishap == number of dice rolled.
* Decrease mishap by spending more WP down to a minimum of 1.
* Mishap 1 => take fatigue == dice rolled. Alternatively, take misery against INT.