As much as I enjoy comparing phones, there is no direct replacement for the N5. Other phones are always missing something, but - to be fair - make it up on other areas.
If you like the size, you could go for the pixel, but good luck finding that one and for a decent price.
The Android One devices look great if all you want is bare Android with updates, but it remains a question how long that will continue. The dev community is non-existent at the moment as far as I can tell, so no root or custom roms.
I have my sights on the Moto G5 Plus as a next device: decent price & specs (memory/storage/sd) and an active xda community. It's a bit larger and heavier and doesn't have wifi ac for instance.
I am getting bored of my N5, but then there's this new flood of roms coming out and you can play with the latest stuff in such an attractive form factor... I can't find an excuse anymore....