I'd love to find caverns/ancient rooms while mining. It would be similar to how you can currently mine out additional tiles by uncovering cave galleys. Little rooms with cool chests/chairs with loot that can only be found inside them.
Some examples could be 'ancient' small chests, stone thrones, ancient stone candle holders, banners with old art on them, and maybe even random pieces of dead trolls in the chests. There are tons of things that could be worked in as possible rewards. Gems for gilding or jewelry also comes to mind.
Haven has always been boring as far as exploration goes. If I travel for hours on any level all I'll see is the same shit on loop.
It would be cool to uncover little secret areas as we play