The GURPS Magic prerequisite system has its advantages. Just a couple off the top of my heard are that you need basic abilities before complex ones, so you don't get guys casting Explosive Fireball who can't Ignite Fire, and encouraging breadth (learn lots of spells) over depth (pick one, maximize it.) Oh, and the fact that "no prerequisites except a minimum talent level" is a cleric and druid tradeoff for a limited spell list.
But the need for some spells drives a repetitive sameness into PCs. My players adore Great Haste, but one player actively avoided it for his PC . . . until it was just a point or two away from purchase thanks to another spell he needed. Still more find that basically every wizard has certain spells and lacks certain other ones, because the path of prereqs is either on the way to something, or is littered with limited-use spells or spells they'll prefer their character was not possessed of or using.
In other words, people want to short-circuit the prerequisite system in my games not to get power on the cheap but to allow a greater diversity of approaches and spell combinations.
Lucky for everyone, GURPS has a solution to this: Shortcut to Power, from GURPS Thaumatology: Magical Styles.
Here is an idea of how to adapt this to GURPS Dungeon Fantasy, under a variant name.
You've learned a spell without going the "usual" route. Replace the spell prerequisites for a specific spell with this perk. It must be purchased simultaneously with learning the spell - notably wizards with Wild Talent cannot use this retroactively (or preemptively) to remove the prerequisites for a spell cast with the talent and then Retained.
If you later learn the original prerequisites, this perk is wasted! This can safeguard you from spells such as Forgetfulness, however.
Gerry wants to learn Ethereal Body without learning a number of Air or Movement spells. Normally, he would need Magery 3 and either Body of Air or six Movement spells. Instead he spends two character points and learns Ethereal Body and takes Alternate Prerequisite (Ethereal Body). He already has Magery 3, satisfying the talent requirement for the spell. Ethereal Body will act as a prerequisite for any later spells, including Phase, despite the change in prerequisite.
Notes: I haven't tried this yet. I'm not sure if I will - or people will take it. It does save points, generally, but it also costs a very valuable perk slot. Ironically, perhaps, by saving points in spells to skip ahead or get a different prerequisite setup you reduce the number of points in spells and thus the number of perks you can have. But it's a nice way to get to particularly choice spells (Phase, Invisibility, Force Dome) without needing prerequisites that don't really fit your character concept. In other words, as much as it's a shortcut, it's one that generally fits a concept not a workaround. I'll see how my players thing about this before I make a decision, but on paper, it looks workable in a game that lacks magical styles as a limiting factor.