I think they can definitely improve the optics and AF speed of the existing model. I used to own a copy but found the 70-200/2.8 II + 2x III was close enough in performance to make it unnecessary to carry around two large lenses. My guess is a new version will get rid of the push-pull, improve the optics + AF, and add the newest IS. Ideally it will also function decently with a 1.4x extender on a body that can AF at F8.
I expect the price on such a lens to be about $3k. The current 100-400 and the 70-200/2.8 I both sold at nearly the same price (70-200/2.8 was a bit pricier). Therefore I wouldn't expect a new 100-400 to drastically exceed the 70-200/2.8 II.
I would certainly be in the market for such a lens and am currently holding some of my budget in hopes they will come out with something (I would jump on a 400/5.6 IS if they came out with that instead). I can't afford $11k for the 200-400, nor can I even afford the 300/2.8 II or any of the other big primes. If I had the money I would certainly buy a 600/4 II, but given my current situation a new 100-400 is a reasonable compromise. I suspect there are many other buyers like I.