I'm probably sticking with my Nexus 4 for a little longer. It does everything that I need and it doesn't lag and slow down like my G2x did. For now.
The one reason I may have to jump on the Nexus 5's successor is if more games gain Android controller support, which increasingly seems to be the case. All that extra performance might seem unnecessary to most, but with more and more advanced and graphics-intensive games entering play store, the extra powerful CPU and GPU aren't just gimmicks.
Still, it would help if more manufacturers would push their phones as gaming handheld machines, maybe offer their smartphones with Android-compatible controllers and a few free games for a few more bucks. I would totally jump on the package if I needed to upgrade from my N4. Imagine Google offering this package:
1. Nexus 6
2. Android gaming controller
3. $20 Play Store voucher
If the starting price for the 16gb is $349, tack on another $30-$50 for the controller, and $20 fort he voucher. $409. Tack on the package discounts you see retail stores for combos, and offer the whole thing for $400.