After considering
a bunch of alternatives including johndallman's suggestion, I think I have settled on "alternative #2.5", as illustrated here:
http://flatblack.wikidot.com/alternative-insignia*.
- Each grade (junior/company officers, senior/field officers, junior flag/general officers) has three† ranks in it, distinguished with one, two, or three stars.
- Junior officers and company officers have no categorical device.
- The categorical device of senior officers and field officers is a crown.
- The categorical device of junior flag officers and general officers is a crown over crossed sword and baton.
- The categorical device of senior flag officers and general officers is a crown over crossed batons on a wreath.
That gives us
- O-1 to O-3: 1-3 stars.
- O-4-to O-6: Crown, with 1-3 stars.
- O-7-to O-9: Crown-Sword-&-Baton, with 1-3 stars.
- O-10-to O-12: Crown-Wreath-&-Batons with 1-3 stars.
This scheme is systematic, consistent, and reasonably simple. It looks kind of British-Empire with its pips and crowns and swords and batons. And bling increases unambiguously with rank.
The problem with this arrangement is that it tempts me to systematic "reform" of rank titles.
*
The illustrations are just conceptual. The star illustrated is the star of the British Order of the Bath; the Imperial insignia will use the star of the Order of Old Earth. The crown illustrated is St Edward's Crown; the Imperial insignia will probably use a somewhat different crown (though perhaps, there being no physical crown, an early artist may simply snaffle a St Edward's from public-domain clip art). The wreath illustrated seems to be of laurel; the Imperial insignia will feature a wreath of oak.
†
The highest rank of senior flag/general officer is purely notional. No such high rank has ever been needed in practice.