I wonder if the actual wizard had any wizardy inspiration. Not that cribbing Gandalf for the Invoker makes him illegal to use as a wizard, of course, but D&D wizards haven't been very Gandalf-y ever.
I also wonder if mearls's experience of fantasy inspiration coming later is something that the rest of the team had, as well -- in other words, that they might start with, say, a mechanical inspiration or a 3e class that needs an analogue, and only worry about the fantasy you can distil from that after the initial inspiration.
Something like "We need a healer who is not the cleric" -> Warlord, or "We have all these neat cleric attack spells that didn't get used" -> Invoker/Avenger.
I also wonder how much their inspiration matches the ideas that come later. Thinking about it after mearls's comment, Gandalf = Invoker makes some sense, but before then, what was Gandalf to you in D&D? And what was the Invoker?