Nice, I've been waiting for a good modern shell to replace ZSH. Fish just wasn't enough of an improvement for me to abandon ZSH. This is an area where most open-source developers have avoided, as it's a "solved" problem, but it really isn't.
I'm currently waiting on Elvish [1] to mature. It's written in Go [2], but they're developing a new language on top of it for shell scripting, which I tried for a week. It's a nice modernization and is incredibly fast but still very alpha.
You don't realize how slow ZSH/Bash is (even without plugins) until you try a modern shell, and there are very few good modern shells around.
Besides performance my primary complaint is just how awful coding in ZSH is. I've been doing it for years and I still feel like I'm running into walls constantly and struggling to write 'clean' code that isn't full of hacks and workarounds.
High performance + modern scripting language = my ideal future shell
I've found the complete opposite to be the case. This is one area where there are hundreds of different shells out there as it seems a "cool" thing to try since it's basically just an extension of writing your own language. Most never go beyond pet projects though.
If you're curious about Go shells, I'm writing one too[1]. It's also alpha but you can already write some reasonably complex scripts in it since it supports message passing, network sockets and background processes. It's definitely not fast though. Fast enough for what I need it to do but there's plenty of room for performance tuning once in beta.