Sorry, I tried to be as brief as possible but at the expense of the coherence of my thought...
What I tried to say, in the case of the Arcane Trickster, is that I would rather be able to pick any of his 2 main abilities (impromptu sneak attack and ranged legerdemain) as selectable feats for arcane spellcasters or as rogue special abilities. The arcane trickster prestige class is nothing really exceptional: it has sneak attack just like any rogue and has bonus spells just like any spellcasting class. What makes that PrC special are the 2 aforementioned abilities. Sure, to balance things, the prerequisites to these feats should at least require the character to be able to cast spells and have some thieving skills.
No matter what, I will never like the concept of prestige classes. There is absolutely no way for the player to get the arcane trickster's special abilities without disrupting the natural progression of his/her character as a rogue or as a wizard/sorcerer/bard. Why would I want to progress as an arcane trickster beyond lv 3 of that Prc if I am only interested in getting a "sample" of his unique abilities? I don't want to force my players into sacrificing so many feats and skill points just to get these 2 abilities.
there is a book called arcanra or arcane sumtin that talks about gestalts, we have 1-2 campaigns going on with that concept and it is entertaining
i think you search for something like that