For me it's a feature combo that's missing. Make a new SE, bump the price up $100, but give it a camera close to as good as the Plus phones, and that's my next phone, for sure. I only got the 7+ over the 7 for the camera, and if there'd been an SE-sized phone with the same camera I'd have bought that instead of either bigger option.

[EDIT] actually I suspect this is a common reason for purchasing larger phones and the phone makers know this, and are using the large-size-better-camera pairing to charge me $300-500 more for the better camera, instead of $100.