If the battery is removable, I don't need it to be larger than 3,000mAH.
It would have to have expandable storage because it needs to cater to enthusiasts. This is not a phone everyone will buy. A factory unlocked bootloader is also pretty important.
5.7" would work I suppose, since that's something the industry likes to make. QHD is a must. I don't exactly need a stylus and I don't think the G Pro line was made for that. But then again, this could be a new birth to the line.
I would like to keep the thin bezels, no curve unless it's minuscule (like the G4).
Front facing speakers would be a no-brainer for this phone.