I'm halfway thru Dr. Wily's castle in MM4 and - wow - I'm impressed. Give this homebrew a try if you enjoy old-school Mega Man games - I don't think you'll be disappointed.
Lots of replayability added to MM4. Works on actual hardware too. Developer Woodfrog has knocked it outta the park. Find it here.
I'm looking forward to MM5, 6, and hopefully some kind of Wily Tower style extras.