Million dollar idea is co-op games that a streamer can play with their chat, at scale. Like, everyone in the chat can log on and operate a giant factory, or command an army, or something.
You'd need some kind of diminishing returns and hierarchical gameplay to get it to work, but as a proof of concept, imagine something like C&C Renegade, with the FX stripped down to accommodate anyone with a toaster and a fixed pool of resources divvied up among a team's players according to the streamer in charge. You could give giant mecha to your half-dozen viewers, or you could spawn in your thousand or so viewers as light infantry and manually pilot a land battleship alongside them, or something. Viewers could get matched into multi-crew vehicles and make friends with each other.
As a proof of concept, maybe some kind of hack of Supreme Commander where every unit you train gets controlled by a different person in your stream.
Minecraft Youtubers had a ton of success with this general concept, starting their own servers and letting the 10 year olds run wild or build megaprojects for their 'king'.