Now, I can't find any suggestion that we should adopt flying at, say, 5,000' Eastbound etc. What happens elsewhere in the World that already has a transition altitude of 18,000'?
In the US it's simple. East is odd, west is even. VFR is plus 500 feet. FL180 and above is Class A, IFR only. In practice Class G is rarely more than 1200 AGL, the rest is Class E by default.
The same rule applies (minus VFR) above FL180 too, i.e. east you would fly at e.g. FL230. This includes RVSM altitudes though my 182 doesn't get up that high. Pre-RVSM the rules are different but hardly something I get to deal with in reality.