Necstasy

Courtesy of Dengeki PC-Engine magazine:

Here's a funny alternate mode that you can access in Download 2. Go to the debug menu (ATS press SEL, (I,II)x5, SEL) and choose to view the ending cinema (#9). At the very end, when "NEC Avenue / SOFIX" are shown on the screen and the music has stopped, press and hold UP+SEL+II and keep holding them (?) until the debug menu comes back on. Choose a valid stage, and all the smaller enemies in the game will be replaced by crazy cartoon characters, like Ultraman, Bakabon, Moomin, etc! Each stage has something weird to see.

This cheat took me a few tries before it worked, so perhaps you should try this with an emu and savestates first.