No one may vote for the Virtual Boy, but I don't think something should be excluded based on the assumption no one will choose it.
I think the market dictates what is a successor and what is a hardware revision or different model. In the case of the Game Boy Color, it replaced the Game Boy, both on store shelves and in game development. The DSi, meanwhile, is being sold alongside the DS, and a majority of its games are still DS games, pretty much all of which are retail games. So, the GBC is a successor and the DSi is a revision. If the GBC is a revision, then the Wii is a GameCube revision.