I'm totally with you! Mafia 1 was my very first game i played to 100%.
unfortunately, there is no public roadmap related to new games coming to Stadia besides the already announced one. You might want to follow the Stadia team on Twitter to stay up to date. (https://twitter.com/googlestadia)
It will but that was delayed for stadia, some source said that's due to coronavirus pandemic, anyway, we wil play it.