I'm not sure what style i used here (i'm bad at photoshop, very sorry)
Here's the list of all used textures to create these icons and their origins if they are from mods:
Tiers

Common - Pearl Block (Grimoire of Gaia 3)
Uncommon - Emerald Block
Rare - Diamond Block
Epic - Zanite Block (Aether II)(slightly edited to not being so dark)
Legendary - Gold Block

Portraits:

Susiria - Angelic and Angelic Light Stones (Aether Legacy)
Mysterious Champion - Lapis Lazuli and Redstone Bricks (Divine RPG)
Ol' Boothilda - Oak Log and Oak Planks
Cardinal Richemelieu - Cake (Bottom, Side and Inner textures)
Snackle - Normal and Crying Obsidians
Davey Drowner - Jungle Wood (Log and Top) and Jungle Planks
Marshall McTaggart - Red Sandstone (wanted to use gears, but couldn't find the texture)
Sarlash - Podzol (Side and Top); Darkwood Leaves, Planks and Log (Twilight Forest)
Dracantes - Prismarine and Prismarine Bricks
Katonnis - Nether Brick and Netherrack
Lady Seldarine - Mossy and Decorative Mazestones (Twilight Forest)
Udunn - Hellfire and Hellfire Light Bricks (Aether Legacy)
Irran-Shu - Orange and White Terracottas; Light-Blue, Green, Red and Purple Glazed Terracottas
Ganda - Gold Block (Prior to 1.14); Golden Aerocloud, Golden Oak Log, Quicksoil Glass (Aether Legacy)
Mycenius - Dirt and Grass Block (side)
Neologus - Iron Block (Very dark to match Neon City), Redstone dust lines (in light-blue/teal colors)
Narandir - Aether Dirt and Aether Dirt Side (Aether Legacy)
Larsene - Thaumium Metal Block, Eldritch Pillar (Thaumcraft IV)
Chief Brosquatch - Snow and Blue Ice
Ambarine - End Portal Frame (Top texture), Ender Stone Brick, Purple Brick
Archlich Kizappian - Cracked Knight Brick, Canopy Planks 0-3 (Twilight Forest)
Uzumi - Carved Stone and Sentry Stone
Yoshimi - Top Furnace Texture; Realmite and Rupee Lamps, White Furnace Top Texture (Divine RPG)

Note:i included Yoshimi and Davey for... actually - no reasons. Because they are not in the Club Heroes squad anymore... but who knows, maybe our "Revangeful burd" will come back soon))

I think i can do the same idea with Rarity indicators without making new style of portraits, just add them somehow.

Club Heroes Portraits