Oh boy, do I have something for you... The Dragon, the Hero, and the Courier
GREGORIOUS Yamada is a hardcore Middle Ages geek. He made a fantasy world that's extremely cool and hilarious, based on actual medieval life, though mixed with the fantastic, like Adventurer parties being accompanied by EXP Trackers, Bards for the victory tune and other things.
What manga do you think didn't live up to it's potential?