I have been messing around with some of C#'s more recent features (like ref structs) to try and make a high performance gamedev library based on relational database concepts.
For ages I've felt like that ECS feels like a more restrictive version of a relational database, so it feels like a natural avenue to explore.
But really I'm just another person procrastinating from real game design by doing pointless engine development instead.
I've found C# nullable types quite frustrating, because nullable generics don't work properly when they can be instantiated with both value types and reference types.
It feels like a very hacky approximation of an option type, which we are now probably stuck with. Compared to all the other amazing features they've added, that one felt a bit half baked.