In my opinion, I prefer a nation wide alphanumeric system excluding Tassie here. I'm not a huge fan of shields, except when it comes to tourist drives and state routes. But when I mean "Nationwide", it needs to be something like 1 and 2 digit routes are M/A routes, 3 digit routes are B routes, and 4 digit routes end up being C routes with a simple solution to just add a 1 to all of Vic's 2 digit B routes, and a 1 in front of all three digit C routes. I'm excluding Tassie here though since it's an island.
But in conclusion, I only like TD and SR shields, and not NR's or Metroads. But alphanumerics is also more easier when it comes to maps and foreigners who'd find a letter and a number easier to recognize rather than different shields.
Oh, and I also don't like bordered alphas anymore