built in terminal with access to (even a sandboxed) filesystem where I can do the stuff I do in the terminal on a mac -- install and use packages (homebrew might work? it's Darwin underneath iPadOS too, right?) and do other bash/zsh stuff; text editors/IDEs (I'd personally like an iPadOS version of vscode); and some mac-only software, like Sketch.
If they had those, I'd switch to an iPad Pro today.