I would expect more use of zoomorphic totems for Angles and Saxons at that time; horse skull with or without skin, or horse tails. Wolf, bear, or boar are alternatives, as would be a sun disk. Draco type standards perhaps, or crude facsimiles of other Roman types. I've no basis for any of this, but anything else feels wrong to me.
Likewise Dracos and vexillia seem right for Romano-Britons, or even an ornate spear or signi trailing ribbons for sub-units. Self-identification as 'Roman' appears to have been a thing, so you'd perhaps expect a display of Romanitas here.
I'd imagine more anachronistic 'Celts' would not be so different to the Germans, apart from use of triskelion-type imagery and severed heads.
I'm not sure flags were in use in the 6th Century, but who knows?
I suppose you'd have to ask the question "What is the standard's purpose?". Rallying point? Religious symbol? Inducement to fear?