Something like this has definitely been suggested before, but I'm certainly up for talking about it.

I like the Standards idea, although I'd prefer them to be independent of the extant Armour Back accessory slot; people pay a lot of money for those things, and would probably rather not sacrifice their wings or sideblade or whatever.

Shield costumes, I'm a little more dubious of; I'm fine with the current costume system as an aid to personalised character design, but something about masking wielded equipment sits uneasy with me for some reason. What if Guild crests were projected, hologram-style, from the surface of the shield, similarly to the Force Buckler's vortex effect or the Volcanic Plate Shield's glowmap? I dunno if it's viable, the way SK's models work, but it'd allow for display of Guild regalia without changing out the shield's model.

Guild accessories and regalia needn't even stop there. What about costume armour?

Here's a quick 'n' hacky Photoshop job demonstrating the sort of thing I mean - (link). For sake of convenience, I've used a Gate symbol for a Guild crest and a Rose Regalia tabard; the helmet's an untextered Replica mask from the Dark Harvest promo. Not saying these would have to be It, but they'll do for a demonstration, as they're both reasonable blanks. Some sort of front Accessory could be a possibility, even; here I've just bunged a resized Defender on the front of a Skolver Coat and chopped the crest in, but you see the sort of thing I mean (standard and illuminated versions).