Put a SD card, more memory.
Just for a start.
I do like the Pixel finger print in the center of the phone.
Like the keyboard also.
Give us more ringtones.
Faster processors.
Better batteries.
Wireless charging.
Brighter screens.
Let the consumer have control over their phone .
Think those would make a big majority happy.
Then work on the software.
Google has only put a microSD card slot on one of their devices, the Nexus 1 from 2010, so that's not going to happen. Samsung put them in all their devices except one (the Galaxy S6 I believe) and they brought it back after outcry from their customers. If Google hasn't done it by now, they're not going to. Also, Google has cloud services where Samsung doesn't, so it makes more sense they'd want people to make use of that.
Ringtones can be downloaded for free from numerous places.
It'll have the SD845, just like every other Android flagship.
Battery size is supposed to be about the same on each device (a little bigger on the regular and a bit smaller on the XL). The smaller battery in the XL isn't drastically smaller, and probably translates into minutes less rather than hours. But I wouldn't say no to a bigger battery.
Rumor is both models have wireless charging.
That's more up to the suppliers than Google. The smaller model has so far used Samsung panels, but Sammy holds back their brighter panels for their own phones. If Google switches to LG OLED panels, they might be able to have more say since they invested around $1 billion into LG's OLED development.
You would think that would cover everyone, but (going by what I've seen them complaining about as shortcomings on the Pixels) it would also need:
IR Blaster
Headphone jack (or 2nd USB C port to allow them to charge and listen to wired headphones at the same time)
Full edge to edge display with no notches or bezels
Fingerprint scanner under the display
2-3 cameras on the back
6-8GB of RAM
Minimum 256GB of storage and 512GB for 2nd tier
All bands for every carrier worldwide
5-6 years of support
Now figure all that in and then figure what that would cost. Charging $1,000 would be cheap with all that included, and a smaller model would be out of the question because there wouldn't be enough room to fit everything in it.
Google has never taken the Samsung approach where you throw everything possible in. Google has been about offering their version of an Android phone. If you like it, fine if not, one of their hardware partners probably have something that will work for you. They've never sold their phones in the same quantities as the rest of the Android OEMs. They've been about offering fast updates and a cohesive software ecosystem for smart home integration, which is starting to come to fruition. If you're wanting all of those things, then it might make more sense to go with a Samsung or LG device, because if Google hasn't changed by now, they're probably not going to.