I love the idea, but I see a problem. If a caster could make up spells on the fly, what would be the point of ever using "Singe Animal". You need to balance this. I would suggest limiting the words known by the caster, either through a chart or a skill check.
EDIT:
Glyph already stated he didn't want this.
woups, teach me to read the rest of the post next time...meh :-/