Well, when Luigi was introduced, he was nothing more than a slightly taller but thinner green Mario, yet no one complained. However, when Waluigi comes in, everybody was bawwwwing their eyes out. However, if Waluigi were to appear in a proper Mario game, it could benefit him greatly, he could gain new powers, like for example Mario is the jack of all trades and can use fire, yet Luigi is a watered down Mario with great jumping abilities and could manipulate lighting. Wario could be the slow but strong guy who uses farts/gas/whatever while Waluigi could be the weak but super agile guy with ice powers. Waluigi could open up a new potenial for a M&L4.