It is hopeless to aim for a suitable CoA for every hypothetical foreign conquest. The number of possibilities is overwhelming. Also, exotic scenarios like Norse Byzantium, Persian France, etc. have no historical models to work from. The results would be nothing but pure fantasy.
But the case of England is special. England is a situation where a foreign conquest actually happened in history, leaving us with a "foreign CoA" i.e. the three lions. But the default state in 769 is that England is an Anglo-Saxon kingdom. In some respects, this never changed. It's worth noting that common people in England to this day are primarily descended from Anglo-Saxons; only the aristocracy was replaced by Normans.
I feel it is reasonable to ask that kingdoms have a "non-foreign-conquered" CoA as their default. It's silly to start out with a foreign one from a conquest that hasn't even happened yet.
That surely depends if *one* generic coat of arms that works can be found, which is more what I meant than one per culture, covering everything.
As I've said before I've rarely seen an Anglo-Saxon k_england form, and more often see something in a Norse vein there.
Norse Byzantium has a couple of possible and plausible things to draw on - the Varangians would be the likely source for such a thing should they overthrow the Emperor, so that one at least is not "pure fantasy" in that there's a plausible method to get there. Not likely, but at least plausible.
Much like the earlier post of Welsh/Scots/Pictish/Irish/Norse "England" are all perfectly reasonable and sensible.
White Dragon is tricky, as it's not necessarily pan-Anglo-Saxon, depending on your source, and could effectively just be one or two of the warlords early on rather than *the* Saxon symbol. It's mentioned in only a couple of sources and didn't carry through as "the Saxon banner" after that - and the sources (with one based on the other...) place it as a mythic allegory of the totem beasts of the Britons and the Saxons rather than any sort of actually representative emblem.
But yes. It would be nice to have a non-lion based England, and if anything the lions should be the exception, with *something* working as the default that everyone else gets.
I don't even like the idea of "generic Norman/generic English" using the lions because it's a dynastic shield that circumstances are unlikely to recreate from the early starts.
Somehow, I just don't like the white dragon on a red banner, and wish there was a better Anglo-Saxon England flag, and perhaps relevantly one that isn't used by a nationalist "revival" movement. As an interesting note, the Bayeux tapestry seems to suggest Harold (as king of the Anglo-Saxons) was under a *red* dragon (the Briton's standard) rather than a white one (the supposed Saxon standard) - although I don't suggest adopting that because it'd confuse matters with e_britannia or d_gwynedd. Unfortunately I don't have a good single suggestion because all the different Anglo-Saxon groups used their own markings and it didn't really get a lasting "national" symbol as such when it did finally coalesce into one state.
That said, I'd -reluctantly- accept it as the 769/867 starting date flag for forming England from the Saxons, and for lack of anything better for forming it with most other cultures.