Well, to toss my hat into this ring ...
Assuming we start with a blank slate:
I always thought a system based on, and modified from, the various navies (specifically the US)would be good.
Assuming we follow the OR-1 to 9 / E-1 to E-9, we'd get:
Rankless uniform for OR-1
1 quarter inch stripe worn on an epaulette for OR-2
1 half inch stripe for 0R-3
1 half inch and one quarter inch for OR-4
Etc. up to four half inch stripes for OR-9
Now, for officers we would follow the same pattern, but with the addition of devices, or embroidery to the epaulette, or a different epaulette.
Some examples of how it could go:
All officers would wear the same epaulette with four half inch stripes running from tip to bottom. Superimposed over the four stripes would be the rank insignia.
Alternatively, we could use chevrons for the non-officers and stripes for the officers, or just use chevrons for both, one pointing up, the other down, and still using the 1/4 and 1/2 inch differences. Similar insignia for different ranks is not unheard of.
I'm generally not fond of using the same ranks, but in different colors, but that's a legitimate route to go. Say, silver for non-officers and gold for officers. Toss in other colors to differentiate the various levels, i.e. enlisted, NCO, WO, JO, etc.
Or, use the same insignia, but in different spots to differentiate ranks. Cuffs for enlisted, on or around the elbow for NCOs, on the biceps for WOs, on the epaulettes for JOs, on the collars for SOs.
Or, have the rank insignia on one side representing one rank, and the same insignia on the other side representing a different rank. A bar on the left collar is a private first class, while the same insignia on the right is for a field marshal.
I'll do a rough sketch in MSPaint of some ideas, and upload them.