I have a pretty extensive rewrite of standard ritual magic called
College Ritual Book Magic.
The basic idea is to treat any individual spell as an Easy skill, colleges are Very Hard, there is no core skill that limits skill, and grimoires provide skill bonuses for specific spells. I've also rewritten the default penalty for many spells: instead of being equal to the prerequisite count, I went through the spell lists and assigned default penalties of 0 to -12 depending on the assumed difficulty of the spell.