Honestly I'm quite fine with the 1520, generally from my experience my friends and family are really impressed with the quality of photos, and the battery life. This is compared to the flagships by the other camps BTW. The only thing is that I can't get the games my friends recommend me, but it's not a deal breaker since I don't play games on my phone. As for everything else it doesn't seem that I am missing anything at all.
The following wall of text is what I'd like from the 1520's successor.
From a hardware perspective for a 1530 I'd want the same form factor with a six inch screen, not a huge of fan of aluminium chassis, with at least comparable battery life. Same camera, in terms of megapixels or more if feasible, but with a bigger sensor. If there's going to be a camera hump you might as well whack a slightly bigger sensor on it, not 1020 big but maybe double the radius of the 1520. Additionally, add an LED on both side of the hump for notifications, make it user configurable even, just make it aesthetically pleasing. Just make sure shooting times are still fast or faster.
Stick to 1080p, and advertise the improved battery life compared to QHD. People woo over resolutions because it's a bigger number, how about ads try to woo them back with a longer battery life number. Incorporate a N-trig stylus with the recent acquisition. As advanced as technology has become, a lot of people still use pocket notebooks and bits of paper to scribble important information. Personally I use OneNote as much as I can on my phone but will instinctively try to find scrap paper if I already have a pen in my hand. Why not leverage the N-trig stylus with new flagships. Change the mentality of "let me get a pen and paper" to "here's my phone and stylus". Got a Surface 3 stylus already? You can use that on your phone too!
Internally, the specs really don't matter if they are recent enough. The snapdragon 800 in the 1520 is still good after two years and I am sure the next CPU will be the same. More RAM, more storage, more CPU. Same story different generation.
Must have dedicated camera buttons, capacitive buttons.
Lastly, a niche and a bit gimmicky want of mine but if possible one or two programmable buttons so I can pause and skip music tracks. Other people might have something in mind on what to do with the programmable buttons. Just offer some generic defaults and let it be user definable. For example I might dedicate it to: a flashlight; open up a specific remote in Unified Remote; open my spouse's People's hub page; open up facebook; et cetera. Point is everyone has that one or two things that they do a lot with their phone that isn't hard to get to by turning on their phone or using voice activation, but sometimes just pressing a button is hard to beat. And give an option for whether to bypass the lockscreen or not.
Well that was my little wall of text for this thread. Any other suggestions from other people on what else they'd like in a 1520 successor?