As for Andre 3000 dropping a solo, for what? It would simply be him dropping a solo for the sake of dropping one just to prove he could do it. Overall, it wouldn't do anything for him and most importantly Hip Hop if it was just simply good and wasn't classic. If you've ever listened to his verses, you'd know he really doesn't care about things like that.
Big Boi drops solo albums because he felt he had something to prove outside of OutKast. He did an interview where he was like people know him more for his role in "Who's Your Caddy" than him being in OutKast. It's a rather outlandish statement (I think XXL even put it in the "nikka Please" section), but it's his words.