You need lightening in a jar...
No D&D clone will ever take off like D&D has, since D&D has carved that road AND has name recognition to boot. You're best bet is to completely avoid the Tolkienisms (elves, pointy-hats, orcs, etc) and try something new (or, since there is nothing new under the sun, a different combination of existing components that hasn't been done).
What that is though, is the lightening. Steampunk? Modern? Savage World? Horror? Swashbuckling? Pirates? Asian? Wuxia? All these have been done.
I quote this one because it summarizes many of the previous posts, and with some hesitation I have to basically agree. The implication of what you are saying is that a fantasy game with the type of popularity I am speaking of couldn't be too similar to D&D, and thus would have to be even less "traditional" as D&D already has traditional cornered, even with the newer 4E elements.
In other words, as someone mentioned the 3E fans will just continue with 3E or play Pathfinder, and the niche game fans will stay in their niche. Some kind of new zeitgeist has to be tapped into; the problem with speculating on what that is, is that it is much easier to sort through the past, when it is always something new, something not yet seen.
And you know what? Something probably WILL emerge, if only for a time. It seems that the Number Two game is always either World of Darkness or some "flavor-of-the-year" that is super hot for awhile, then drops back down (below WoD) with a solid, if diminished, following. Exalted seems to be the most recent instance.
How much market share did Exalted grab when it came out?
Good question and I don't know.
The thing with RPG market share is that it is hard to ascertain in that it overlaps as gamers tend to play different games at different times. So you can't really say that "60% of gamers play D&D, while 25% play Wod, 10% Exalted, and 5% everything else" because some of those D&D player are also playing Wod AND Ars Magica AND Buffy AND...etc.
But my sense is that Exalted, for a very short time, was the 2nd most popular RPG out there, then it fell back down below Vampire and/or WoD as a whole. I could be completely wrong, though.