So, say you're running Grimpa the 20th level magic user. Grimpa's high up on the totem pole of the Order of the Green Hand, so it's her turn to help make a Wand of Magic Missile (we'll be using LotFP:WFRP rules for this exercise). The wand needs 99 charges; that means that Magic Missile needs to be cast into the wand 99 times, all at once. Grimpa, though, only has eight 1st level spell slots. We've got a problem. Either she's is going to need a whole lot of scrolls, which take a long, long time to make (and are expensive!), or she's going to need 12 other 20th level magic users to help her out.
Or does she? She's not going adventuring today. Her colleague Melmok, who has the unfortunate propensity to accidentally open gates to The Place Where Large Things That Kill You Live, is gone for the month. Her plans for the day include some pleasure reading, checking the library to see whether any lower level Guild members need any help and eating out with a couple friends. There's really not a need for any other spells. What about Grimpa's 37 other spell slots? Could she use those?
Which brings up a good question: can magic users fill spell slots with lower level spells?
I'm thinking of three pretty good answers for this.
1) No. That's not the way Vancian magic works. Everything has to fit exactly. You can't put a 9mm bullet in a .45 chamber and expect it to fire, can you? Don't complain, you get to break the laws of the universe every day while your buddy Thog over there only gets to bash skulls.
2) Sure, why not? Fill your spell slots with any spell that is of the appropriate level or lower. If you really want to cast Magic Missile a few more times instead of Web or Knock, what's that going to hurt? Now Grimpa only needs two colleagues to help her make the wand.
3) Yes. Also, you can cast more spells if they are lower level than your spell slots. You can fill a spell slot with 1.5 spells that are one level lower, 2 spells that are two levels lower, 2.5 spells that are three levels lower and so on. So Grimpa can cast three Magic Misiles with two 2nd level spell slots, two Magic Missiles with one 3rd level spell slot, five Magic Missiles with two 4th level spell slots and so forth. With this system, Grimna can cast her 99 Magic Missiles and doesn't need anyone's help in making her wand. She can even cast the Permanency spell that's also necessary for making the wand; all told, she could prepare 114 Magic Missiles if she wanted to.
What do you think? Is there another good answer to this question? I personally like the third answer the best. I doubt this kind of thing would be used by PCs very often, but I'd be interested to see whether they would in fact use this at all. It feels like this would open the door up for more creative spell use, but is it too lenient?