Modern? Airspace Classes?
This entire discussion is amazing. It points out the folly (and unnecessary extreme complexity) of our current CNS/ATM system globally. All to just get from A to B, or from A back to A, while keeping Vehicle 1 from hitting Vehicle 2, or from any vehicles trying to occupying the same space on a runway at the same time, whether in IMC, or VMC, or unknown MC (e.g., at night or between cloud).
If Wilbur and Orville (or Pearce, Whitehead, or Clement Ader) had to deal with all this, ...things like ICAO and various state versions of Class A, B,C,D, E, (F), and G airspace, and an inch thick set of airspace rules, they likely might have decided to never invent the airplane in the first place!
The bottom line is that this entire discussion points out the serious need for all of us, globally, to re-rethink what we're doing, and how, and why, with separating air vehicles, and with coping with all-weather operations at a more fundamental level. Just with Class E alone, there are pages and pages of pros and cons that will never be definitively resolved.
So while there may be some very useful ideas, in what Australia has contributed (e.g., FANS and a simple version of ADS-B), and some useful ideas from the US (e.g., TCAS, RNP, GLS, and early data links), and useful ideas from Scandinavia (remotely operated facilities), ...it is time to fundamentally re-baseline all this ATS evolution globally, recognizing that no one country or system, especially not the US ATS system, has a corner on the market of good ideas, or for that matter, on weaknesses. Both NextGen, as well as PresentGen, need massive attention, review, and evolution, as does SESAR, before any of these should be considered as any model for any other state.