Here is another optional change to GURPS Magic.
Per GURPS Magic, p. 10, it costs 1 FP per ongoing spell to cancel it early.
It doesn't exactly specify if this is a free action, or takes concentration. We've always rule it takes a Concentrate manuever, since it's a 1 FP cost and that's extremely unusual for something that would count as a Free Action.
But what if you have a lot of spells up?
We've started to think about ways to do this. Here are two options we're considering playtesting.
To cancel an ongoing spell early, take a Concentrate maneuver. For one spell, success is automatic and costs 1 FP. You can attempt to cancel multiple spells, but there is a risk. Roll vs. IQ, with a penalty of -1 per spell you wish to cancel. You must specify which spells are being cancelled before you roll. If you succeed, you remove all of the specified spells and pay 1 FP each. If you fail, none of the spells are cancelled. On a critical failure, ALL of your spells are cancelled, at a cost of 1 FP each. On a critical success, you may choose after the fact to cancel additional spells beyond the first - however, the FP still remains 1 per spell.
Another option is IQ+Magery, but -1 per spell up, no matter how many you want to cancel. Have 10 spells up and want to drop 2? Roll IQ+Magery-10, and if you make it you remove 2.
A further option is -1 per spell you don't want to drop or -1 per spell you want to keep, whichever is higher, which inverts the penalties and says dropping one is easy, all is easy, but picking and choosing is hard. Have 5 spells up and want to drop 2 or 3? -3. Want to drop 1 or 4? -4. 5? -5.
Why not Will? Because Will is pretty cheap, and I like the idea that its a question of magical power (Magery) and intellect (keeping tabs on all the things you are doing) or just intellect and not a question of your force of character.
I'm not sure which one we'll try, but it's fairly certain will will try one of these. I'll run some test case numbers when I have a chance.