Ooh, now that is a good question.
I suppose for me, I would like to see a greater increase in local / national mythology.
Whilst we do have a few in the UK (We recently got King Arthur & Loch Ness Monster 10p coins as part of the A-Z), I'd love to see more in that style.
Things like the monsters Lambton Worm, the Kelpies, Black Shuck, or any number of wizards, warlocks, crones, druids and the like or heroes like Beowolf, Robin Hood. We have such a rich tapestry and I think it's a shame so much of it basically is forgotten.
I know the Royal Mint has been churning out bullion with some of those “big name” topics, but we just don't get to enjoy them in day-to-day usage.
Some would probably translate really well onto banknotes as well where there's just so much more space. Imagine a King Arthur one with maybe 3-5 of the most famous myths. Or pair them up with real-world locations (e.g. Tintagel Castle or Glastonbury Tor).