Net 5 looks great, but it's unlikely to land in Unity any time soon. Unity runs on its own fork of mono, and if you're trying to build for mobile you'll need to use the il2cpp backend. Official support is for some versions of C#, not all of dotnet (eg, F# and VB are not supported).

I haven't played with it yet, but I think Stride3d is an open source engine that's actually built on dotnet - I'd expect newer versions would land earlier and wider language support.