Fun Fact: Mega Man 6 featured the first two Robot Master villains designed by Americans who had entered the design contests run by Capcom.
Mega Man 6 is noteworthy as one of the last good games published for the NES. Capcom decided not to publish Mega Man 6 in North America as the Super Nintendo Entertainment system was already out, so Nintendo had to take over and publish the game themselves.
By this time the mechanics were so refined that there wasn't too much room for innovation. A new Energy Balancer automatically refilled the Master Weapon with the least amount of energy when Mega Man picked up power for them, and rather than Mega Man mounting other characters or support items to get to inaccessible parts of a level, now they directly attached to his body.
The story offered nothing new, so Mega Man 6 was as good a time as any to close out the saga of the franchise on the NES.