I'm not really up with all the latest things that all the flagships have in them, but I'd imagine there's a bunch of things that they could do that would fit into the "spirit" of the GH line, being that each one isn't really head-line grabbing but cumulatively it creates a real workhorse.
Things that come to mind (but is by no means everything):
PDAF with all the modes (face, eye, dog/cat/gerbil-eye-AF)
Variable eND
External RAW support (BRAW and Prores RAW)
Focus breathing compensation
Shutter angles
All the prores modes internally (including the 4444 12-bit mode)
Dual native ISO with a nice high second ISO for serious low-light performance
Support for more than 2 channels of audio
Clean HDMI out
Updated BM-like UI where you can choose the resolution, frame-rate, codec, quality settings, audio settings etc all in one simple place
1000/1500/2000+NIT screen
They could also make an effort to create a good package / rig, by offering products like:
Updated interface module that offers XLRs, TC, other stuff
On-camera hotshoe shotgun mic like the Sony ECM on-camera mics which make a nice compact package
Bolt on ARRI LPL adapter
Integration with DJIs system so the camera can talk to the gimbals (for follow-mode) and their LiDAR products so you can have AF on manual lenses etc
A custom-designed grip that provides extra batteries but also integrates a swappable SSD NVMe (the long skinny ones) so you can record to SSD without having to mess with cables
I mean, there's nothing life-changing in the above, and we're not breaking the laws of physics by suggesting a 20-stop DR from an MFT sensor, but if it came with half of that stuff then it would be a serious offering I'd say.
I mean, if you think there's no improvement above other offerings just watch a few videos where people list the 27 reasons the FX3 isn't a cinema camera, or the equivalents from any of the other brands too. No camera has all of the good features, they're all missing a random smattering of them.