I got my Nokia N900 more than a year ago. While at first I was not happy with some issues, I currently have to admit that it gets the job done. What could be the most interesting improvements about an hypotethical N950?
Battery: its battery life is less than stunning. But, man, this only applies if you are "far from USB" more than 4-5 hours per day.
Computing power: it is not actually an issue (and you can verify it overclocking from 600MHz to 1150MHz: almost double the processor speed, almost same user experience. Yep, it's true that the "safe overclock" to 850MHz). The real issue is the core memory. 256Mb RAM is more than enough for preinstalled software but, as soon as you begin to install all those Linux ports (originally meant to run on some juicy-powered Linux PC), you will feel the N900 slow. Having -say- 768Mb RAM and 256Mb swap instead of 256Mb RAM and 768Mb swap would make the machine fly as an eagle.
Extra multimedia hardware: please, please, PLEASE! Stop talking big about HDMI and such stupid stuff. We don't need every day video output. More megapixels camera? Are you sure? I have to resize to 30-35% every photo before shelling 'em on my blog or on some friends' email. If you know a little bit about digital photography, you will understand that quality does not mean "megapixels"! A reflex Nikon D70 has the same megapixels of the N900 camera: what is the difference between the old D70 and the new N900?
Hardware improvements: many people complained about the USB connector. I'm sure I over-stressed my N900 USB jack for more than one year... without -to date- any consequence. USB3 support? Huh? When you're in a crowded bus, how and why would you connect some USB3 device? And what device?
Generally speaking, most of the improvements could be done in software. For example, realmedia decoding is a bit slow (because it doesn't use the DSP).
Talk about talking big...! ;-)