A long time ago, I fiddled around with ideas for an FRPG system in which spells were free-form rather than taken from a stereotyped list. My question is whether there is a system out there like this today.
The idea was something like this. A spell would have a difficulty level, which would be computed on a point system. For example:
How much volume does the spell affect?
the size of a fist ... 0 points
the size of a person ... 1 point
the size of a dragon ... 2 points
How much energy does the spell expend?
similar to a candle flame ... 0 points
similar to a running person ... 1 point
similar to a running horse ... 2 points
How much does the spell affect the consciousness and perceptions of others?
subtle visual impression ... 0 points
makes them fall in love, change political parties, etc. ... 5 points
There could be other kinds of points for other things, e.g., duration, points if you want the spell to not require concentration to maintain, negative points if you are willing to have the spell's effects vanish after the spell is over...
The obvious design issue would be that it would be hard to balance the system appropriately. A possible way to deal with this would be to make all people and creatures magic to some extent, so that most uses of spells would become battles of skill. E.g., even if you just want to cast a spell to put the watch dog to sleep, the dog has some magic resistance because it's seen magic before.