50% more battery life than the 4, fanless options for the m3 and i5 versions, on screen support for the surface dial, and a new hinge that lays a little flatter seem like the big changes. It's largely an incremental improvement.
Fanless i5 would be nice - my sp4 i5 isn't loud, but quiet would be better.
Was hoping the new sp would be waterproof though (it's the only remaining "obvious" improvement I see. We'll that and enough gpu power to drive vr - but I suppose we're still a few years from that in a small, light, quiet battery-powered device).
It would be pretty damn surprising if you got any more than an incremental improvement this late in the game.