They'll need to put at least 6gb of RAM, a larger battery, a better camera (both hardware and software side) and better software to make it worth 500. If not, they should just stick with 400.
$500, depending on the spec and the design of the phone, I might consider... Definitely seeing the trend of flagship price going up.
I would not want a phone for $500 or more. That's just ridiculous. In the end, a flagship cell phone is a luxury for me. I don't actually need to have all the bells and whistles. If ZTE can keep the price at $400, I'd give up water resistance, fingerprint scanner, and/or the metal unibody design. Hell, if they left out the last one, maybe they could make the Axon 8 with a user replaceable battery! In that case, I'd be willing to give up quick charging, too.
500 to 800 Bucks for a phone you will be looking to replace in a few months because of the next big thing, is kind of insane.
Its almost like replacing your television every few months. If there are means for it then go for it, yet for most of us that is not the case.
I try to keep under 300 to 400 and keep the device for two years at least. But that's just me.