I was thinking about how you have nothing to show for your hard work after every world reset and if there is something nice but fairly trivial that could be added like badges or something but then I thought what about some sort of decorative furniture like a skull altar or cabinet where you have all the skulls of your previous world characters laid out. Or portraits that you can hang on the wall like "ancestors" showing appearance. Of course they should visually show the highest achieved stats somehow or its pointless (assuming that data is available) like the amount the skull glows or in a hover tooltip or just in examine. Just seems sad to have nothing to show for progress, even if its a trivial decoration thing.
Another idea I just thought is you could have the hearthfire itself be slightly visually improved by 1 tier for every world you have played and achieved a developed skull character so long term veterans might have a really cool looking hearth fire.
It could also be a motivator to not miss a world too because people care about dumb stuff like this way more than they should.