Dr. Light created the Mega Buster by reverse-engineering Heat Man’s weapon data.
- The Atomic Fire is the first, and for the first 3 games, only, Charged Attack weapon. This ability would definitely come in handy, so Dr. Light worked to adapt the charging system that Mega Man got when he copied Heat Man’s data to work with the standard Buster cannon. It took a while, which is why Mega Man didn’t have it in Mega Man 3. Ongoing tweaks and revisions are why its form and power vary so much across the series. It wasn’t until the Mark XVII X-Buster that he was able to make a universal charging system that could charge the Buster and any copied weapon.
Meta WMG, Top Man's stage in 3.
The data for Top Man's stage was lost when somebody tripped over a power cord, erasing all of the unsaved data. The developers had to make what we have now, which is a more greenhouse-themed level than a top-themed level. However, the original concept for the Doc Robot takeover had every stage be revisited, each having one MM 2 Robot Master as opposed to two Doc Robot fights each for four levels. The Wood Man Doc Robot was going to be in Top Man's stage. It wouldn't be hard to assume that the plant theme was somewhat related to Wood Man, but the only major issue is it's slightly off the mark. (Top Man's stage is general plantlife and glass while Wood Man's stage involves a lot of trees.) Dark Man III is the basis for Vile's design in Mega Man X.
The arm cannon was adapted into Vile's shoulder cannon and his specs improved upon to make him much more dangerous and independent.