DDO would fit the bill nicely. Although it's an MMO, the mechanics and the instancing make it fairly solid for solo and/or duo gaming. I haven't seen any instances of being hounded by strangers, so you shouldn't have people disturbing your coop. Last, it's got what I'd call a dual learning curve. Just "playing the game" is very easy to do—I've got an 11-year-old that's never heard of D&D and has no clue what rpg is all about, and she's plugging away, happy as a clam, with a toon that's a certified disaster. At the same time, the character development has immense depth so experienced gamers can do a ton of min/max or customization to their "vision of the toon" as they see fit.