There were going to be two originally, before they decided to turn her into half an aircraft carrier
. The Brits plonked the left-over 18" guns (the other Furious gun and a spare) on a couple of monitors, where it didn't stand out at all
Prior to the Washington Treaty nixing future development, both Britain and Japan had serious plans for 18" armed BBs in the 1920s (not WW1, but more than a decade earlier than the Yamatos - although Britain's N3s would have had to get past some budgetary hurdles).
That said, it's also important not to roll with the "a gun is defined by it's size". The 16" guns on the South Dakota/Iowa class were better guns with better shells than those on the Colorados (or Nelsons/Nagatos).
That said, don't get me wrong - some kind of ship-designer style approach for ships would be awesome
. However, rather than have arbitrary caps on technology being installed on what hulls, have some kind of weight/capacity restriction and hull-size approach (although this is what I'd hope for, I'm not suggesting for a second it's what the devs have in mind) - so if someone wants to go the full Jacky Fisher and whack 18" guns on a light cruiser, but have no capacity left for decent armour, then go for it
.