My two cents:
1. Significantly improved battery life. Oh what I wouldn't give for 10 hours of use, but even honest 20% would make a difference.
2. Bring back a Touch Cover and include it with the device. On my SP2, I used it more as a screen protector. It's great for those times when you are mostly using your SP as a tablet.
3. Two USB ports, wouldn't mind type C.
4. Reduce the number of issues - make it reliable...
5. And finally cheaper, especially the higher tiers...
Honestly, I find the form factor and design to be close to perfect on the SP4. Battery life and reliability are the two big minuses.