The one thing that would truly change my life is a true all day battery life, iPhone 11 style. That means even under load (Photoshop, Final Cut Pro, etc. for at least 10h a day + and extra 8 hours of writing, web browsing, 4K Netflix Apple TV streaming, etc.).
Homo Sapiens has turned into Homo Rechargus with all the stuff we're constantly trying to be on top of in terms of battery management and as far as I'm concerned if I could simply have to worry about this only at night while I sleep it would be most ideal. Even more so now that Magsafe is gone (I'm a klutz so having to charge it only when I know I'll never trip on a wire because I'm basically busy sleeping would be a godsend).
A better screen would be welcome. To me 14" would be nice but 13" so as well. It's rather questions of screen uniformity, manufacturing sample variation, backlight bleeding, colour accuracy, colour space and compatibility with calibration systems that concern me. Increased contrast with Mini LED is always nice to take... as long as the manufacturing sample variation is under control.
The other thing I'd love to see, baring Apple succeeding where the industry has failed for more than a decade and finding a truly exceptional form factor for a tablet / laptop hybrid with little opportunity costs compared to a dedicated laptop (ie no thicker, less powerful, or more fragile), would be for the MBP to support a new Apple Pencil 3 in hover mode with the trackpad, like a Wacom tablet, and with the ability to snap the pencil on the side to charge it. Only a minority of users will benefit from this but from a design perspective it can be achieved with little opportunity cost for other users (from an engineering point of view though IDK if hover will be easy to implement).
Face ID would be nice, particularly since the tech can actually be used for more applications (better detouring during video conferences to blur the background for example). Touch ID will always be bound to be just an ID system. The introduction of the neural engine in the Apple Silicone would enable Apple to just port over the Face ID system without much software effort... provided it can fit in the thin screen enclosure.
On the other hand knowing that my new MBP is 2mm tinnier than the previous one, or that it's 50% faster instead of 30%, I don't care that much. Or even that it's got a Touch Bar or not (I actually like it, but wouldn't miss it much either).