I love Tkinter, but I could accept letting it be an installable package rather than built-in. I can certainly see its size and complexity being a burden on smaller installations. I think making "pip install tkinter" be bulletproof would go a long way. You're not going to start using Tkinter unless you're reading a tutorial anyway.

There's always going to be a good debate about which batteries should be included. I'd almost put numpy ahead of Tkinter, but maybe that reflects my bias for scientific programming.