[Galaxy Note 4 General] sauron0101:
I think we've reached a point now where the marginal improvement is going to be lower for each generation.

Moore's Law has ended. The cost per transistor for SOCs is lowest at 28nm. 20nm will mean higher costs per transistor. At the same time, the marginal performance gain per node will probably drop, while the capital costs for the next node are rising. EUV and 450mm wafers are problem plagued. This in turn, limits performance potential. That means only architectural improvements are possible. Perhaps they could make bigger dies, although this might cost more. That said, with margins for smartphones still high at the high end, large dies might be practical.

Cameras are probably at their limit now for reasons of physics. Only way to make them better is to make them bigger, which is what Nokia has done. It will never rival a DSLR though.

There's limited room for improvement in terms of performance too because of heat and battery life.

On that note, one area for potential improvement might be battery life. It will mean transitioning from lithium ion batteries though.

Speakers might be an area for improvement for Samsung (something like what HTC has done), but otherwise there's limited room for improvement.

Internal storage may benefit from 3D NAND, but it seems like SSDs progress is slowing, so it's inevitable that phone progress in this front will too. SD cards will also be affected.

We will probably see some improvement in terms of display technology. Amoled is now high end a pixel density that the Pentile is not a drawback, and perhaps the Blue subpixel will be given more life. Perhaps we will see better daylight visibility too and more efficient Amoled technology. This might improve battery life somewhat. Otherwise, we are near a very good display - high enough PPI not to be annoying, bright enough to be relatively visible in sunlight (according to Displaymate), well calibrated, and with lower power consumption then the previous generation.

64 bit processing might lead to more performance and take advantage of a lot of RAM. It'll be a while though before software takes advantage of it. Heck, most mobile software cannot yet take advantage of all 4 cores on a phone.

I could see some things like water proofing get added as well. But beyond that, what else is there on the horizon?

I agree to an extent. Perhaps in its day it was arguably the best phone that could have been realistically been built.

Amoled was pretty good for its day, especially in movie mode and the Pentile because it was partial RGB, was not a huge issue despite the low pixel density

Good camera, with only the Nokia camera phones coming out on top

Pretty good battery life Wolfson DAC, providing very high quality audio out

Pretty good speakers

Exynos SOC was the best for its day, although Apple did with its PowerVR series have a stronger GPU

Qualcomm radio was pretty good at holding a signal, as was quality of call.

Only flaw was that Samsung did not release full documentation. The Exynos problems have hampered AOSP development.

No other major flaws (ex: like the Galaxy S and the GPS problem).

For it's day, it was perhaps the best phone that could have been made.