If the new zoom lens is only as good as the 20 year old 400mm f/5.6, then I'll give it a miss as it won't be much better than the current - we want something better.
Some of us would be perfectly happy with the same optics, but with up-to-date AF and IS. Faster & better AF is welcome, as would be 4 stops of IS. That lens is 15+ years old technologically
I don't expect leaps and bounds better IQ from the new version (gotta leave something for the f/2.8 primes and the 200-400... ), but somewhat less vignetting and corner sharpness would be fantastic.
I, for one, don't particularly care about twist to zoom or push/pull, so long as I can still fit it in my bag!