We currently have flags for that though, Vibe. They're fine imo, just without the clans banner.
But such flags would be nice of course, if you can buy them in the shop they are starting with your own banner ingame. But I dont think that the banner should change later on but stay the same. 1 Slot should be enough imo. Ofc unsheathable.
In big events that could be fun! 10 clans vs 10 clans, 200 men, each with their own banners.