Don't get me wrong, I do like the usage of Mega Man music (of course, since you ARE playing as Mega Man), bot the only thing I would rant about is the MM3 title screen BGM in the daytime overworlds. It starts a little too cheerful and slow for my tastes, especially for the most-seen level variants in SMBC. Maybe if we gave all the overworld levels of each World (excluding 3 and 6, as they are nighttime) a small set of randomized music that changes for each time a world is played (like how it cycles through the characters' music options if you set it to "Random", but only with Mega Man music. Or maybe if we could decide on a new track to use in the daytime levels. Dunno how to work it out.
Excellent. I also thought the overworld music was inappropriate. Actually, I thought all the music was inappropriate except for the castle music and the night music.
It's also been announced that Proto Man will have his own Music Set; so perhaps that'll make some more people happy. Personally, Mega Man's Castle theme always got on my nerves after a while. You sort of dislike the song when every boss uses it. As for a non-tedious boss theme that people would probably like better, the first thing that comes to mind is Mega Man 7 - Wily Stage Boss, but I'm not sure how Jay and Zach would feel about having 8-Bit covers of non-8-Bit songs in the game...
Actually, I thought the night music wasn't in perfectly the right place - wouldn't Crash Man work better for night?