I've been talking about complicating the magic user class (here and here) by giving them possibly unlimited use of cantrips (0 level spells) but requiring the cantrips as prerequisites for higher level spells. Thus, to cast Mirror Image you'd have to have some kind of zero-level bend light cantrip (the first link above goes into that).
Rob Conley of Bat In the Attic suggested in a comment that I check out GURPS because the magic system does just that - it's progressive - to cast the really good stuff, you have to learn the really basic AND intermediary stuff. Very cool.
So I read through the magic system in GURPS 4e Magic (as well as the magic chapter in GURPS 4e) and I really like it. As I was reading I was thinking that I'd like to just bolt this on to AD&D - tweak it a little and then call it the magic system. I'd toyed with point systems before, so this wouldn't be that different. And GURPS (like 2ed) divides spells into colleges, which is okay - I think that I could deal with that, too - just have to brush up on the rules for non-college spell use. And GURPS has some really cool spells, too - so that's a plus - either bolted on nearly as is, or as a resource to mine for AD&D spells.
But it all kind of came crashing down on me when I saw this in the appendix:
That's when I realized that it wouldn't just be fiddly - but downright complex. Now, I recognize that GURPS 4e has something like 1,000 spells to deal with, so that's going to make the flowchart even more complicated and 1ed AD&D mages only have 260 (counting UA) spells (+66 Cantrips, though I'd be rewriting nearly all of those if I went with my system) and Illusionists 92(+8) - so AD&D is less complicated to begin with...but it might not be as easy to systematize those...352 spells (and 74 cantrips) in the way that GURPS has (NOTE 2ed has 312 magic user spells - divided into schools - cantrips are handled way differently than in 1edUA). NOTE, too that none of the above (AD&D) includes reversed spells. So...maybe it's not quite as uncomplicated as I thought at first...
So, I'm kind of back at the beginning. I really like AD&D (actually I like it as is but I'm a consummate tinkerer, so I can't help myself) - and I like GURPS magic - but I don't like GURPS rules in general (I like archetypes - character classes - ability scores - saving throws - etc the way they are in AD&D).
So I'm going to explore my ideas a little further - GURPS is a great reference for how to handle the idea of prerequisite spells - but I have to decide how UN-AD&D I'm willing to go. The whole original idea was similar to what James M posted here - only to reverse engineer it - so that it's a kind of "at will" ability for spell casters who know certain spells - not just when they have them memorized, but at just about any time - maybe limited in some way (like# of uses = INT + level or something...).
So - with GURPS sitting open here, the secondary effects from Trollsmyth (for B/X, but a start) and my PH, UA and 2ed PH open as well - I'll try to see what I can come up with...