Well; to maximize backwards compatibility, lets add all of them.
I think adding a few characters is no big deal for Jan, but doing complete character sets for Greek and Cyrillic is something completely different.
If we really want this to happen we should consider paying him for this work. He already made a new "professional" version of Kaffeesatz named Kava with support for many languages, but under a non free license.
For sure adding whole character sets in a proper way would need weeks of work. I don't think he is willing to do such a big amount of work for free - even if he supports our cause. So we won't get it if the FSFE isn't willing to invest money in that. I'm not even sure if he would do it for money. I'm under the impression he thinks, that his non-free fonts are more advanced and he doesn't really want to invest a lot of effort on Kaffeesatz any more because he sees it as an early step, which is not as good as his newer achievements.
He offered his help on the condition, that we do not want to change anything, but just adding some additional characters. The reason for that was basically, that he would prefer having one complete master source for exporting all available versions. Many font versions with different character sets in varying quality are for sure nothing anyone should want.
So if we really want to have Yanone Kaffeesatz in Greek and Cyrillic, I guess we need to find someone else to do this. I'm interested in designing fonts, but I'm just a beginner with limited time. Additionally I don't know Greek or Cyrillic. So I'm far from being the best person to find proper outlines of characters in "Kaffeesatz style" in those character sets. Beside that it would take me much more time than someone experienced like Jan.
Best Franz