I would love a phone which is has ~4.5" screen, 1280x800 maximum resolution, could do with 720p, but a bit bigger in that dimension is nicer, has a great camera, has generally flagship level hardware such as CPU's and RAM, but doesn't go silly in forsaking battery life. Battery life is the big one (apart from the form factor) where I think it should be able to last at least a whole weekend with moderate to heavy usage, without a charge. Obviously more is better, but I guess it's good to be realistic. Oh, and since it's a thing now, still have the headphone jack. Apart from that, all the other things are standard features, wifi, bluetooth, GPS, etc... so no point going through everything.
Now, that's just the hardware. With regards to software, I'm torn. I love the experience of WP8.1, it's a really nice clean UI, but lack of support is killing W10M. There's just no way that the platform can continue as it is, unless either some killer apps come its way, or big apps start supporting the platform.