Working for a carrier, I see a lot of phones that do all kinds of quirky things. There are some features that are awesome, and others that are weird. I'd say for me, my perfect phone wouldn't need to be anything super thin or light. I'm fine with a brick, as long as that weight is going toward a great battery. LG G5 and V10 are pathetic with battery life. I have a second phone that's a G5 and the battery drains faster than the Galaxy S6, which was terrible as well. Give me good battery life. If the screen needs to be a slightly lower res to achieve that, I'm fine. 1080p and 1440p are not that far off for me. I like the new phone displays that are super high res, but I also don't like watching my battery percentage drop so rapidly.
I also like the fingerprint sensor concept. Iris scanner is great for Windows Hello, but maybe not always the quickest way to go. I like the placement on the back of phones like Nexus and LG. It's easy to hold and unlock my phone. But if the phone is sitting on the table, you have to pick it up fully to unlock with finger. Apple had a pretty smart idea putting it on the front, and it is ergonomic as well, but it also takes up vertical space on the phone, making the phone a little taller. Pros and cons to everything. I'm fine with either way. Just don't go the method of the Moto Z and put the sensor on the front and also not utilize it as a home button. It confuses people.
Camera is probably what I am most picky about. I live and die by good Lumia cameras. It would take a lot to convince me a non-Nokia/Microsoft phone could be better. But I really want a quick shutter speed. iPhone and Samsung are great at this. LG, too. Windows Phones are super slow. I miss so many great moments because of the slow camera and how long it takes to process photos.
A unique design would be awesome, but not the most important feature for me. But it will be in order to win over new users. Everyone likes an attractive device. It increases perceived value.
Expandable storage is not a must, but if it isn't available, 64GB would be idea for internal storage, just to be safe.
I don't need a gimmick like cartridges or magnetic back plates. If the phone will be modular, it needs to be intuitive and supported long term. Moto Mods are cool, but no one is really buying them. LG "Friends" never took off at all. If there is something modular, it would need to make sense. No need to jump into an idea just because it is cool. It needs to be sustainable and scalable.
Not something WhatrtonBrooks can control, or any OEM, but the inconsistency and glitchiness of Windows 10 Mobile needs to stop. I encounter problems daily, and it is frustrating to see common things not work well, or see Android phones or iPhones doing simple tasks when I'm struggling because my keyboard decides it doesn't want to backspace when typing an email, or even send it after pressing the send button. Small, but very important details like that need to change. That's on MS to fix.
That's all I can really think of for now. I see a lot of gimmicks on phones, daily. People want a phone they can rely on and do what they want it to when it needs to. The fundamentals come first. The fancy stuff can come later.