I know this was already an idea posted along with other ideas, but I want to isolate it and expand upon it.
My idea is simple, have a customizable surface (portrait orientation for banners, landscape for flags).
I know you can do this with photoshop, paint.net, GIMP, Krita, etc. and then the in game posters, but you can't blueprint posters. Something like this could probably be copied via blueprints. Also this is the biggest advantage over posters.
In my opinion, Medieval Engineers did a good job and so did Minecraft. But when I see those, I see potential. Maybe have some features comparable to games as old as Tony Hawk American Wasteland's Create-A-Graphic (I loved it btw) or as new as For Honor's Emblem Designer. I haven't really played the latter, but the first you are able to move symbols around and change their color, size, and alpha, as well as flip on both axes. This feature would be great in a banner/flag creator.
When I say symbols, I mean symbols. Like vector graphics. Nothing with shading or gradients.
I don't think there should be custom symbols unless they too are blueprintable, since that's the main reason of having banners and flags. I mean you could have it where if i were to blueprint a flag with a custom symbol on the same server or world, it would work fine, but if i were to blueprint it to a different server or world, the flag would just show up as an empty image. Like the red IMAGE MISSING dds file under Misc.