A side power key would be my first choice. The Desire 820 has one so I'm hoping they bring it over. I never touch it with the Dot View case on, but out of the case it is a bit of a stretch, and I would leave the case off more often if I could hit it without having to shimmy my finger across.
I didn't say more megapixels, because it's too much of a common sense/natural progression change. They aren't going to stay with 4MP for the third year in a row, and I remember an interview with one of their camera engineers saying that they thought 8MP is ideal, and that's where they wanted to get to. I personally don't want anything over 13MP right now; 8MP alone is usually overkill. When you start reaching 13+ the only real benefit is cropping, and that's because (in most cases) optical zoom isn't there for varied focal lengths. (So try to find a way to include another zoom level without drastically increasing the size.) What I'd want from the camera is the purple fringing gone, and more consistency. The camera isn't terrible, it just ranges from great to pathetic. I can take three pictures of the same thing and 2/3 will be acceptable, but all three will look different.
The black bar doesn't really bother me either. I like having someplace I can rest my thumb. The screen is so sensitive that I can actually hit the on screen buttons from only the very tip of the black bar. If they got rid of it completely I can see myself accidentally activating the buttons all the time. I'd like it if they made it smaller; if they did something like fit a slightly larger screen in the same height and a smidge wider, but I don't think it'll be gone completely. It seems most likely that they'll follow the Desire 820, and make all the bezels a smidge smaller, rather than chunk out a whole portion they've been using for two years, and X amount of devices.
Something less likely to happen, but that I'd want is a dedicated camera key. Having one is just so nice. Being able to just press it to go into the camera would be so much easier than having to hold the phone landscape and press a volume key, which is both slow and never works unless I'm holding the phone perfectly horizontal. And you could do it with the screen already on, since trying it right now just ya know turns the volume down.
Probably the only other hardware change I'd like to see is an improvement with the speakers. "Make them sound better" or "make them louder," which would be great, but that's not what I mean. When you get further and further away from the speakers, the quality of what you hear drops off, and that's what I'd like improved. Physics is limiting it, but make listening from 10 feet away closer to what you'd hear from 4 feet. It sounds great up close, but the bigger the distance, the more hollow it seems.
Thought of something else: have some sort of grip or texture on the sides. The phone is slippery, and even though it's gotten less slick as time went on, it's still slippery. They don't have to mess with the entire phone, just the sides you clamp your hands or fingers on. The lines on the sides could be extended to cover the area inbetween and given some texture without redesigning the whole thing. Or it could be styled like the Droid DNA grilles. Some really good color combinations could be made from that instead of all blue or all red.
With software, having Motion Launch to turn off the screen would be great.
More themes, or being able to customize the colors would be good too.
And please for the love of all that is good with gravy, take away the clock in the status bar if you have a clock widget, or at least the stock HTC clock widgets. And in the lockscreen. It's redundant to have a clock in two places at once. Nexus 7 doesn't have two on the lockscreen. You don't need two. Open up that space for something else like more notifications or so it doesn't look like an entire row of crowded icons.
And most importantly, keep what made the M7 and M8 great. Except make them better.