There's a lot of potential in the 1.15 updates to do cool things with vanilla banners, here's one idea: New Banner Patterns. They would probably be sold by traders for each faction. These banner patterns would be used on the Loom to add mod icons to vanilla banners which will allow you to make your own custom banners. Here's a few ideas, feel free to post any you think of:

Gondor: White Tree, Seven Stars, Crown of Númenor, Dol Amroth Swan Boat, Anórien Sun, Ithilien Moon, Seven Small Stars (Ithilien Banner), Lossarnach Rose, Lebennin Shell, Lebennin Waves, Pelargir Anchor

Rohan: Horse 

Mordor: Eye of Sauron, Morgul Skull, Morgul Moon, Nan Ungol Spider, Black Uruk Fire, Black Uruk Cleaver 

Lothlórien: Circle Emblem

Lindon: Diamond Emblem

Rivendell: Four-Pointed Star Emblem

Woodland Realm: Mirk-Tree, Corner Designs 

Dunland: Wolf

Isengard: White Hand

Angmar: Iron Crown, Rhudaur Axe

Near Harad: Near Harad Snake, Umbar Double Snake Staff, Gulfing Snake, Gulfing Sun, Nomad Sun, Nomad Snake

Iron Hills: Seven Stars, Durin's Crown, Anvil & Hammer

Blue Mountains: Triple Blue Mountain Peaks, Seven Small Stars

Dúnedain of the North: Dúnedain Star

Dol Guldur: Dol Guldur on the Hill

Gundabad: Eye, Triple Mount Gundabad Peaks 

Half-Trolls: Half-Troll Face

Morwaith: The Great Lion

Taurethrim: Mûmakil Head, Pattern at the top

Dale: Bow & Arrow, Esgaroth Fish

Dorwinion: Wine Cup

Rhúdel: Star & Square Emblem, Small Corner Designs

Shire: I have no clue, maybe a Smoking Pipe?

Bree: Bree House, Bree Tengwar, Design at the top

You would be able to combine different banner patterns together on backgrounds of the main 16 dye colors, which would lead to unique patterns and designs which would be very useful on servers. Let me know if any other patterns should be added, and what to do for the Shire.

Credit to TheArcReactor for the idea.