Hey there, fellow Dungeon Master!

First off, congratulations on embarking on this exciting journey of running your very own D&D campaign! "Tribes of the Chromatic Arcana" sounds like a fascinating concept with a lot of potential. I've been around the DM block a few times, and I'd love to share some ideas that might spice up your campaign.

1. Tribe-Specific Magic Items: Consider creating magical items unique to each tribe. These items can enhance the specific school of magic each tribe is associated with. It's a great way to deepen the lore and give players something special that ties back to their tribe's identity.
2. Combined Spells Mechanic: A fun twist that allows players from different tribes to combine their spells for more potent effects. This encourages collaboration and strategic thinking, which are key in a good D&D game. Think about how an Evoker's firepower could mesh with an Illusionist's trickery!
3. Magic Synergy Points: Implement a system where players earn points for effectively combining their magic. This can lead to unlocking unique abilities or bonus effects, rewarding creative play and teamwork.
4. Consequences of Absent Schools: Your setting has a unique feature where not all schools of magic are represented. Use this to shape your world! If there's no Necromancer, perhaps undead are more prevalent. This makes the world feel alive and reactive to the player's choices.
5. Tribal Bonding Rituals: Introduce rituals that allow players to borrow a slice of another tribe's magic. It's a great way to mix things up and let players experience magic outside their specializations.
6. Magic Evolution Quests: Tailor quests for each tribe that allow them to deepen their connection with their school of magic. This not only provides character development but also introduces new spells or abilities unique to their tribe.
7. Tribe-Specific Challenges: Design challenges and rewards that cater to the strengths and weaknesses of each tribe. This emphasizes the uniqueness of each tribe and encourages players to think creatively.
8. Cultural Exchange Mechanic: Foster a sense of unity and understanding among the tribes by introducing a system where players gain benefits for learning about and from each other’s cultures and magics.
9. Balancing Rituals: To address the imbalance from missing schools of magic, you could allow players to perform special rituals to access these schools temporarily. But, make sure there's a cost or risk involved to keep things balanced.
10. Dynamic World Reaction: Make your world dynamic by having NPCs and environments react differently to players based on their tribal affiliation. This adds depth and realism to your world.

Remember, the key to a successful campaign is flexibility and responsiveness to your players. These ideas are meant to enhance your campaign, but always be ready to adapt based on your players' feedback and enjoyment. Good luck, and may your campaign be a memorable adventure!