There's also The Super Ninjas, which also contains huge sprawling maps with plently of treasure and secrets to find and is a vast open world as well. It's also batshit insane, with the dialogue, especially NPC dialogue, being stream of conciousness like. Expect plenty of Real Ultimate Power references.

I haven't finished it yet though, it's pretty tough, with you not being able to grind for money and medicine being a limited resource.