Ideally, what features and changes would you want to see in Python 4.0?

Proper tail-call optimization! I know Guido doesn't like this, but proper TCO enables you to write elegant solutions to many problems, and I think that far outweighs a marginal improvement to ease of debugging. You can still debug TCO'd code just fine; a stack trace isn't your only tool.

Support running in a pendantic mode requiring and enforcing type declarations.

https://stackoverflow.com/questions/63838471/possible-to-enf...

Also, simplify the whole str / byte / utf-8 encoding decoding process. Python is the only language where these errors are so prevalent.

Backward compatible with 3.x or a migration module which magically makes it compatible. The migration from 2.x to 3.0 was long and painful. I'd be fine if 3.x just continues (Perl is at 5.40 now).