Mama mia! This was great-a!
Hey hey hey! I don't usually review popular/great movies, but since browsing through reviews I see no ideas (or no serious ones) as you had requested, I wish to suggest one now. But first, I make this review short and sweet, like your movie.
The humor was great, a couple of laughs in the actual movie, and the outtakes brought it to a full. Graphics and animation was quite smooth for a sprite movie, which amazed me. I like your choices of music (the music from SMRPG and funny music video too) and sounds from the original game as well. Overall, Mario movies never cease to bore me, and this was a great addiition to the collection already here on Newgrounds. So you deserve this 10 I hand you, along with a 5 to top it off.
Now, the idea I have for what Mario did to Luigi: he used Luigi's bathroom and pooped all inside it! Of course, don't actually show the poop, but in the flashback, have Luigi walk into his bathroom casually with his bathing towel, and as he walks in, he drops his towel and toothbrush and stuff and you hear a loud scream as it zooms out to the full view of the castle, where Mario is walking out of the front door whistling. And this is where it ends flashback and cuts back to the action again.
Please use my idea, I think it would be great for some laughs. You can edit it as you like. Thanks, and keep up the good work. No matter which idea you use, I'm sure your next piece will be just as great, if not better.