Maybe we could keep a general 'scrolling' sub-genre. Axelay has both horizontal and vertical levels .
You know I never thought about that. Also, I'll have to add that game to my shmup collection soon!
how about
Compilation / Shovelware
Add-on/Expansion pack
Coin-Op Conversion
Anime / Manga
Shovelware wouldn't be considered a genre, it's more of a term used to describe generic budget titles, usually with poor production values. Those games still have their own genres, and should be listed as such. And if they are in a combo pack, then they should be listed as a compilation.
Add-Ons and Expansions aren't genres, they are a type of release, the expansion themselves have the same genre as the original game. For example, using our current list; Diablo II is an Action / Hack 'n Slash. So it's expansion pack, Diablo II: Lord of Destruction, would also be listed under Action / Hack 'n Slash.
Coin-Op Conversions should prolly have their own listing alongside dedicated cabinets, since some arcade games were only released as conversion kits, rather than in their own cab.
And I'm not sure what you mean by Anime / Manga. For example, games like Fullmetal Alchemist: Dual Sympathy, Naruto: Ninja Council, Jump Super Stars and Ghost in the Shell: Stand Alone Complex are all based on anime and manga, but they are all completely different genres.