If vinyl can come back...
Seriously though, all that really needs to be done is to make modern OSes run older software better. Having classic Window interface options would be nice too.
I don't miss a CRT monitor, the things were physically huge compared to LCD. Image burn-in is much less of an issue as well- I know LCD can suffer mildly form this(mine currently is) but I've seen old CRTs that have the image burned into the screen so bad you can see it when the thing was off.
I suppose keeping older style connector ports wouldn't be bad but SATA > IDE in performance. IDE is really only still being made for legacy hardware users who need them for their old W95 and earlier machines, and if modern Windows could support that software better those old PCs wouldn't really be needed.