You can't make bosses as they are coded elements of the game, I think if you could draw your own boss sprites then animate and script the boss to meet your requirment then that would be pretty awesome.
I've noticed there are certain obsticles with numbers with symbols on, I take it these are scripts? If so does anyone know how they function? Only one I think I know is P which I think stands for particles.
I'm not really too fussed about it but I noticed the game has bars above and below the screen, as far as I know when you have bars in game it's because the game is playing under a widescreen aspect ratio which shouldn't do (I'm using a 17" LCD monitor....I know, 1280x1024 isn't the best reso compared to what the world has introduced but games still look amazing with it). I tested SMB on my spare PC (Which is connected to a widescreen TV) and it was in full screen perfectly so it just makes me wonder if the game is running in a widescreen aspect ratio.
It's also very anoying using devmode as I have to move my cursor well away from buttons in menu's to click anything because of this.