On my web site,
http://www.bigfoot.com/~D.U.Thibault, there are a couple of GURPS Grimoire addenda, which could both be adapted to GURPS Magic for 4th edition with some work.
SJG only contacted me as the project was nearing completion, so I was unable to have any input whatsoever in the new edition. I would have liked to add at least the Sympathy enchantment and the Spoil Magic meta-spell. And it would have been really useful to put in the Spell Variations system. The reason there is an Explosive Fireball but no other Explosive <missile spells> is the Variations. They were intended to allow GMs and players to systematically create such variant spells. Explosive Fireball was kept because it was grand-fathered. Likewise the Mass Sleep/Daze/Suggestion and Invisible Wizard Eye/Ear spells. Ah well, maybe the fifth edition? :-)
For the record, I was responsible for about 60% of the new spells between the original GURPS Fantasy book (ca. 1986-1988, included the magic system plus the Yrth background) and the first edition of GURPS Magic (1989). And then I oversaw the first draft of GURPS Grimoire, for which it is hard to evaluate the proportion of "original" versus "submitted" --it was a large scale collaboration, based on hundreds of pages of bulletin board (pre-Internet!) discussion threads. I like to think all of those spells bear my mark, to varying degrees. Reading the fourth edition, I see I've left a lasting legacy, and I'm happy for it.