Go for it, Daniel!
I think you’ll find that handling references to local & instance vars is the fly in the ointment. Method-missing doesn’t cut it since it’s very low precedence compared to a real local var.
That’s my biggest worry as well. I’ll give it a shot - I have an idea for how I can cut the problem into smaller steps.
Actually, the assigns didn’t seem to be a major issue. I’m struggling a bit with the output buffering though.
Can I start a Work In Progress PR on GitHub? I have a few ideas for how to proceed, but I’d like to get some feedback soon 
By all means!
Yup, if you have some code, a WIP PR is a great way to do it.
Okay, I’ve started a PR at https://github.com/rails/rails/pull/10807
It’s still a bit in flux, but I’d like to understand how output buffers are handled. Since each rendering is now scoped to a single object, there’s a lot less to keep track of.
I also still have to copy some stuff over from Template, so it’s very early.