Well, no, actually I was referring to a problem I have with (almost) every level 20 build I've seen:If you mean a wizard becomming a Thaumaturgist. Malconvoker qualifies you.
The character may have cool abilities on level 20 but in order to get there he sucks for 19 levels and would never even make it to level 20 if you had to actually play through those levels.
Take the nightmare spinner: Is that prestige class any good before you get its key ability at level 10?
Since the OP asked about a level 6 character, I'm just not sure if providing a level 20 build is helpful. At least I'd have liked to see a level-by-level guideline.