I think that using a Lego graphic library to create a game could be quite nice. Probably would be easier than creating all the graphics by hand, and it would give a distinctive style to your game.
However, if I see a Lego game, then I expect the game to involve assembling and creativity in some way. Maybe we could we create a Lego based character system? Something where each level nets you a new piece which you then use to assemble your character sheet, which in turn defines your abilities? For example, on a level up, you might choose a horse piece. So, by putting your character on the horse in the sheet, you would give yourself the riding ability (equip a weapon to gain the mounted combat ability). By putting your character next to the horse, you would gain the animal empathy ability. Also, you would also be able to combine different pieces in some ways. For example, you might put a horn in the horse to turn him into a unicorn (so his meaning in the character sheet would be different) or maybe put flames on his hoofs to make her a nightmare (I know this wouldn't fly with the normal Lego horse)
Or maybe you could give the assembling aspect to the player by allowing him to interact with the world in that way. Crafting could require the player to mess with the pieces in order to create what he desires. IF he wants to make a potion, he needs to disassemble the ingredients, take the relevant parts and put them in a vial in the correct order, so the pieces connect correctly. Furthermore, you could also manipulate your environment as pieces. For example, a dragon model whose mouth is stuck with a rock so big it can't further open its won't be able to use bite attacks. Then, by providing means to use the environment to his advantage, the game would become more "Lego-like".
