My own action MMORPG (think Mordhau meets Cyberpunk meets Arma 3). It's the perfect application of everything I already know as a platform engineer, and I get to learn all the things I don't. I'm making the client foss, the assets foss, and the gameplay compelling as all getout. Non-sharded, persistent world, with different lands for different real world regions. It's a type of metaverse in truth, but some of that part I have to flesh out better on the local client side where you can do whatever, but on the server there is a storyline.
I almost applied to YC because I'm at the stage I'm close to public alpha and need funding, but instead I'm planning on crowdfunding, but the release strategy has to be tops. I'm also doing things like planning on how to scale the business itself, lots of work on the over time growth profit model, etc. So basically, instead of a thousand side projects, I have one giant project where I get to do everything with my own theorycrafting - after years of being stuck doing whatever the boards/c-suite needs, it's a taste freedom and a dream.
Been working on it since 2013...