Make it so you gain access to extra dungeons, the dwarf place, and the elf place (each with their own shops and side quests)
maybe a path opens through the trees at the north into an open overworld like MoonFable.
Just add more areas, dungeons, shops and items that become available after you beat the game. If the bad guys are defeated, why should stone basin still be closed? Doesn't make sense. Maybe different character types should be given perks, like dwarves have more health, humans have more attack and elves are balanced, with a gnome expansion, allowing you to visit the gnome traveller and trade with him whenever you want.
And Silverwind's technically retired, but maybe he'd make an extra sprite or two for the extra bits.