Ian Thomson wanted a more "free form" sorcery system, which doesn't use
fixed spell lists. May I recommend the GURPS magic book as a source of
inspiration if you have a spare tenner or so to splash out? Some of the
improvised magic systems in there might fit the bill. Also first or
second edition Chivalry and Sorcery had elemental magic with no fixed
spell lists which might help a bit.
As a caveat, C&S magic was too bloody complicated for most mortals (such
as I) to understand, so it might be more effort than it's worth!
Ash