Nope, I don't really remember that.
Maybe you remember this, but I actually tried to get other people to make levels for Bowser's Last Stand 1. Since nobody seemed interested, I ended up deciding to make them myself, and that was even the whole purpose of the "rapid-release cycle" (to pressure me to get the levels done, because otherwise I was taking ages to finish them).
Chances are, yes, the difficulty will by taken up. Bowser's Last Stand was supposed to start out easy to establish Bowser's power; BLS 2 doesn't need to do that as much.
I probably wouldn't have been interested early in the development cycle though since I didn't know if the game would even have been completed.
But later on I must have forgotten.
Can I help with the level design for the sequel? I have a tendency to blow a lot of my gimmicks pretty quickly, so how quickly I get burnt out depends on my ability to overcome that. I'd still like to help though.
You could also check raocow's talkhouse for level designers, I'm sure you could find a few since they're working on a SMW hack at the moment anyway.
How would you implement the Mario and Luigi fights - I assume they'd be bosses - but bosses usually shouldn't be more agile than you since it leads to a lot of cheap deaths.
I don't want to spoil too much (I have some really great ideas for BLS 2 or else I wouldn't have even considered it), but it will involve fighting more than just toads; also Bowser's minions, Mario and Luigi, and if I can get it to work right, Yoshi. There's a legitimate story behind it kind of similar to a video on Newgrounds I really liked (but with a connection to Bowser's Last Stand).
I think Giga Bowser seems more like a temporary powerup to me - it would just seem a bit weird to be Giga Bowser for an entire level.
I think it's also not too much to mention that I'm hoping to add Giga Bowser as a powerup (and that the intro will somehow explain that Bowser thought of the idea of Giga Bowser from Megabowser). I'm not sure if I should make it a temporary powerup that makes Bowser invincible (kind of like Mega Mario in NSMB), or a more permanent powerup that does something else.
I'm considering that too and it would be an easy thing to do, but I'm just not sure where to do it...
I'll look into it.
Just put in some bullet bills and some spikes, you don't really have to balance it as much as the main campaign - so it should be relatively quick to do.