Definite: Newer cpu (probably Tiger Lake). This would include AV1 hardware decoding on any gpu. By 2021 intel is expected to have a massive IPC jump for golden cove, but by then who knows if Apple will be on ARM or not.
Almost definite: Wi-fi 6 (802.11ax)
Seems likely: Mini LED screen
Unlikely/would be nice to doubt it'll happen: More ports (HDMI? SD?) and have the touchbar killed off. High quality front-facing camera. At this point it would almost be bad to ruin their hilariously bad streak of horrible cameras.
Separately: It would be great for going from 16gb of RAM to 32gb to not cost $400 f*ing dollars. Actually more since you can usually find the base models for like $200 off on sale but not custom builds, which means that 32gb of RAM on a macbook actually cost $600 more than 16gb. Equating for the fact that you also lose 16gb by upgrading, it's like paying $700-$800 for 32gb of ram. Come on Apple.