I remember always dreaming of a multiplayer M&L game ever since my first moments playing through Dream Team. What I didn't know was that there already is a multiplayer M&L game: Superstar Saga. At least, it kinda sorta is when played on a GameCube or a Wii U.
Connecting two controllers to either system when playing GBA games will register them both as P1. Not sure whether this was actually intended by the devs, but in Superstar Saga, this essentially creates a multiplayer experience as long as one person uses specifically the A Button and the other uses specifically the B Button.
Various walkthroughs utilizing this have been posted on YouTube, and soon I want to try playing the game with my brother to see how well it works. I imagine Bros. Attacks will require a lot more skill and coordination then previously, especially considering that my brother has not played the game before. But whatever the case, I think it's really cool that the option for multiplayer was there from the very start of the series, even if it might not have been intended by the developers.
In Super Paper Mario when Luigi flips to the right his emblem on his hat gets flipped too. This goes on till he is captured and turned into Mr. L. After you rescue him in the Underwhere it's fixed. I find that to be so strange.