My dream spec for the Surface 4 would be:
Core-M processor - ditch the Atom completely or keep it in the base model and higher models use Core-M. The Skylake Core-M CPUs seem to be very impressive.
2x USB-C ports
1x USB 3 port
Full size SD card slot
Improved keyboard and larger trackpad with better feel (much like the improvements that were made to the Surface Pro 4 keyboard/trackpad compared to the Pro 3's).
Magnetic stylus (so it sticks to the side of the screen)
Reduced screen bezel and increased display resolution. Keep the form factor of the device the same size.
Any reduction in weight but battery life maintained or increased is always welcome
Windows Hello looks cool but I haven't been able to try it on a computer yet.
Ditch the 2Gb ram option. 4Gb minimum even on base model.
8Gb ram option, along with storage capacity going up to 256/512Gb.
Basically, Microsoft can you please apply the same improvements you did for the Pro devices, add an extra USB port or two and have a Core-M, 8Gb ram, 256Gb storage configuration. That spec with the stylus would be lovely for editing images on the go. I really like Windows 10 and combined with the above spec it would be my portable computer heaven