Roll20 is system-agnostic. While some game systems can get complicated and could require an API script (a Pro subscription perk) to get the mechanics right, most games can be played completely for free. And many RPGs have playable dragon, part-dragon, or dragon-like races. The various editions of Dungeons & Dragons, for example, have a few different playable dragons scattered throughout, although they're often poorly balanced. A better draconic PC race in a D&D game is Dragonborn (a race of humanoid reptiles closely related to dragons). I know D&D 3.5 also has the Dragon Disciple prestige class, which essentially turns a non-dragon Sorcerer into a Half-Dragon over the course of 10 levels. Other games use dragons, as well. Fireborn, for example, has dragons living in modern London. Exalted second edition has Dragon Kings, which are similar to D&D's Dragonborn in some respects, but with a unique magic system for the setting. (No word yet on whether DKs will be available in Exalted 3e yet, but the core book was only released relatively recently.) And of course you can always create custom content for a game. I recall playing a World of Darkness game once where one of the PCs was a dragon (custom race), but humanity's general disbelief in his existence muted his power-level. Also he was a thaumivore, so he had to fight with mages and werewolves over demenses for the magic he needed to survive.
