Had this idea for an achievement the other day when I was thinking of playthroughs I could do.
Name: King Arthur
How to achieve: As Wales or Cornwall, conquer all of England, Wales, and the Lowlands of Scotland.
Idea behind the achievement is that the native Britons (Welsh/Cornish/Bretons) have managed to successfully defeat the Anglo-Saxons/Normans and Scots and reconquered their original land. One of the legends involving King Arthur is that he would return and do exactly that. Idea comes from that legend. Achievement could be done as Brittany as well, as they are also Britons.
This image roughly shows what would be needed for the achievement. Fully coloured provinces shows what would be needed to earn the achievement, whilst the lined province in the same colour shows what the game considers to be part of the Scottish Lowlands (may or may not be needed for the achievement). Lined provinces in Britanny shows what it would be like if the achievement was done as Britanny. Lined provinces in the Highlands and Ireland shows what would be needed if the requirement was to conquer all of the British Isles as Wales/Cornwall/Britanny instead of just the native Briton land (some legends depict King Arthur as having established an empire over the entire British Isles, as well as Gaul, Norway, and Iceland).