Thanks. First, we would need to decide what would earn a reward and then fill in all the blanks with reward ideas. I took a look at the banner section on the Terraria wiki. There are 258 banners total broken down as follows:
Pre-Hardmode: (87 total)
- General List: 64 banners
- Slime varients: 16 banners
- Critters: 7 banners
Hardmode: (80 total)
- General List: 73 banners
- Slime varients: 7 banners
Events: (91 total)
- Sandstorm: 6 banners
- Goblin Army: 6 banners
- Old One's Army: 10 banners
- Pirate invasion: 6 banners
- Solar Eclipse: 14 banners
- Frost Legion: 3 banners
- Martian Madness: 11 banners
- Pumpkin Moon: 5 banners
- Frost Moon: 9 banners
- Lunar Events: 21 banners
Based on this, I would add rewards for the collecting each of the following:
(rewards in green)
- All 258 banners: The Unification Catalyst (material that is the final ingredient to a very useful accessory that is the result of merging several accessories earned through Found Girl rewards)
- All 87 Pre-Hardmode banners:
- All 80 Hardmode banners:
- All 91 Event banners:
- All 23 Slime banners:
- All 7 Critter banners:
- All 6 Sandstorm banners:
- All 6 Goblin Army banners:
- All 10 Old One's Army banners:
- All 6 Pirate Invasion banners:
- All 14 Solar Eclipse banners:
- All 3 Frost Legion banners:
- All 11 Martian Madness banners: Martian laser cannon (end game laser)
- All 5 Pumpkin Moon banners: Halloween Sigil (reusable: sets game time to the Halloween seasonal event, use again to turn off and canceled out by Christmas Sigil)
- All 9 Frost Moon banners: Christmas Sigil (reusable: sets game time to the Christmas seasonal event, use again to turn off and canceled out by Halloween Sigil)
- All 21 Lunar Event banners:
- Nymph banner: Nymph Summon Staff (great idea by Horrior)
- Pinky banner:
- Doctor Bones banner:
- Tim banner:
- The Groom banner: The Bride banner (there isn't one in Terraria)
- Mimic banner: Mimic Summon Staff
- Rainbow Slime banner:
- Paladin banner: Paladin Armor set (very high defense armor the debuffs your speed: idea by Horrior)
- Wyvern banner: Wyvern Tail (accessory that significantly increases flight time)
If you want more rewards additional groups could be added such as: Dungeon enemies, Rain enemies, and Biome specific enemy collections.
Some banners are a part of more than one reward.
I usually create the missing corrupt or crimson biome in my world but some people may find that tedious so maybe just one set of enemies could be required for group rewards.
I'll try to think of some more. Also, some really good ideas have all ready been suggested so maybe some forum members here can help fill in some of the rewards...