I'd like to have Dark Souls and Mount & Blade. The child would be Dark Souls camera system and lock on system, with Mount & Blades sandbox world and weapon variety. Dark Souls would lose the magic and high fantasy world, but keep the graphics.
Second I would mix Mass Effect with Titans Quest/Diablo (never played Diablo, but TQ is a clone). I keep the Mass Effect sci-fi world without the story, its graphics, it's third person shooter style, and overall feel. It gets Titan Quests' never-ending loot system and seemless gigantic world. The story takes place in the Titans Quest style with occassional dialogue from NPCs that pushes you further on your quest.