You also don't have to have a sheet to play a system. Many people play with sheets created as free text or tables in the bio section of a players journal, they can then manually create attributes if they want to use macros etc. There would also be nothing to stop you from starting a game in this fashion and then collaborating with the sheet author to make your manually created attributes the standard names for the attributes on a sheet which would mean that once the sheet was live most of the information would already be filled out for your players when they next logged in. Worth noting though that you would need a pro-level player to build the sheet (or at least be able to test external code in Roll20) and have it added to the list of community sheets to use it as a free account holder OR you could upgrade to pro yourself and use custom code in the game settings to apply the sheet (this gives the added benefit of allowing you, the game creator, to enhance play with the other pro benefits which your players would also be able to access within your game. Ultimately though, which ever course you take, there can never be enough community created sheets for an agnostic system like this :)