one thing i think would be interesting is to find treasure in the dungeon chests to decorate your house/fortress/base/whatever. i know we can find rare items like apples and records in them, but they are extremely rare to find... (found 4-5 dungeons and mostly got buckets, wheat, bread and pig saddles).
here is what i think that could be found in them:
1) statuettes (maybe depicting the different mobs of minecraft. id love a golden creeper statuette XD)
2) rare paintings (basically have a unique painting on it that is otherwise un available)
3) ancient armour/weapons (for display only, or have very low hp)