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