You can play any game you want to via Roll20, Roll20 is as agnostic as your dining room table is when it comes to 'supported games'. Even if there isn't a fully statted out character 'sheet', you can still make your own 'sheet' via the Bio and Attributes & Abilities tab in a character journal. If you ask politely, someone will likely assist you in creating a character sheet. The games dropdown menus exist to help standardize the options available
(example: 4E vs. 4th Edition vs. D&D4, etc). No matter how many
games are listed, some may not make the cut. Games are added to the list
periodically. This ensures obscure listings gain greater visibility
when sorted via the Other Games option and helps keep away “empty” search listings for underrepresented titles. If
you feel a game should be listed, encourage your community members to
play and post often. The more community attention a title receives the
more likely it is to be added to the list.