Rather than creating actual "levels" and specialized mechanics for Prestige Classes... I think the more sensible method would be to make Prestige Classes be entirely "story based" design, and then it tells you what existing mechanics in the game (skills, feats, multiclassing) you could select to "create" it.
It's basically the same recommendation I had when the talk about "fluffy" Fighter subclasses occurred last year. For that, it was basically "Write up the fluff of what you want-- say a Gladiator-- and talk about what the Gladiator does, what is important to it, what it focuses on... and then give a recommendation of the Battlemaster maneuvers and feats you should select to exemplify that fluff mechanically."
For Prestige Classes, it should be the same way. Come up with a story for this Prestige Class-- what does it do, who would it be important to, how does once become one-- and write up the fluff of it. Then, using multiclassing rules, skills, feats, spells and the exchange of class features as necessary (like how the spell-less ranger took out spells and replaced it with maneuvers)... create examples of how you could build the mechanics for the five levels of this Prestige Class.
The reason why this rarely happens though... is because more often that not multiclassing is used not from a story perspective but rather an optimizing one. 20th level characters are "built" to maximize their work for whatever job they are being built for at 1st level. Story progression during the campaign itself usually has nothing to do with it. A PC doesn't have Sorcerer and Warlock levels because the story worked out that a innate magical character somehow found and made a connection to some extraplanar being and made a deal for additional power during the campaign... instead, the PC has Sorcerer and Warlock levels because it was built from the beginning to have those levels to maximize their Eldritch Blast cantrip with metamagic and the like.
Usually if a PC multiclasses "mid-game" due to how the campaign just happened to play out... optimizing is the last thing that happens. And quite often the PC actually ends up being less-powerful because the levels of the new class do not necessarily "synergize" with the existing one (and indeed, the character would have been more powerful mechanically had they not multiclassed at all.) Which means that Prestige Classes of this type would not result in "power creep", because usually the character would end up less powerful mechanically (white room speaking) than they would have otherwise.
Instead though... they become optimized in another way: in the story. Story-wise they are more powerful because they gain via the fluff much more than they just get with the mechanics. Becoming a 'Lord's Alliance General' (for example) garners them more power within the story than any extra numbers they get on their sheet when they roll a die.
Prestige Classes that are there just to give out "new mechanics" cause the problems. Prestige Classes which symbolize the evolution and power of a character within the confines of the story give us a reason for their existence. So create the stories, or create the templates for individual DMs to layer them in to their own stories. And then give a few examples of how that can mechanically be presented using the current methods already available in the game.