Like many, I am hoping for some optical improvement of the new MK. III TCs, in particular the 2x TC.
However, I wonder how much potential for improvement there is actually left. Optically, TCs are probably quite simple, further, contrary to lenses, there are no moving parts, so there is also less potential for QC problems which a new version could help to get rid off.
After all, what a TC does is an optical magnification of the central portion of the base lens' image - no matter how perfect this magnification is, it will always also enlarge any flaws of the base lens.
What do you think, how far are the Mk. II 2x TCs away from "perfect", i.e., how much better can a Mk. III even be?