A buster only challenge like in 9 or 10 would have worked. I'd have personally gone for defeating the Yellow Devil in the original game with just the default buster weapon and stuff like that, but maybe I'm just a glutton for punishment when it comes to these games!
I did like the boss challenges in the Legacy Collection though, while I don't agree with the not giving you health back between fights (making the Air Man battle rather evil since his attacks are practically impossible to dodge) I did learn a great deal more about boss patterns for many bosses so its not been a waste. Still, I never want to do the buster only, all robot masters challenge again (at least until I get the Xbox One version).
I don't really like the Mega Man 1 one since you have to use the Pause Trick to get Gold.