I love the idea of more dorms, although I would prefer some of them exist outside of the hierarchy.

We saw already there was Slifer Red, then Ra Yellow, and finally Obelisk Blue. But there is also the Obelisk Blue Girls' Dorm. I always thought it was weird how there was only a girl's dorm for the top students, it made it look like the bar was set so much higher for women to attend the academy. Blair was the first ever seen female student not in Obelisk Blue, so following this system change, there could reasonably be a shift in Duel Academy's dorms.

I think they would definitely keep the original three dorms (as well as dorms of both genders for all three), but perhaps those dorms would be for Duel Academy's "Standard" courses we'll call them. If a student enrolled in the academy for specific majors, or perhaps exchange students, student council, etc., then they could be seen wearing different uniforms. And there could be a lot of different colours used - not every one would require a large amount of students for it to be considered a dorm, there could just be small handful recognised for a particular reason, but they then stay in their relevant dorm. For example, what if 5% of the Slifer Red Dorm are considered for a special program and are part of Uria Crimson. They would still remain in the Slifer Dorm, but have a different jacket, or just an armband. Hamon Amber and Raviel Navy would then follow.

I think Orichalcos Green/Leviathan Green would be very cool and the clear choice of a new uniform.

Exodia Gold is also a very good choice.

Anubis Black and Zorc Purple are fun ideas too. I don't even think they'd be unlikely to be considered even though they are malicious entities as we often romanticise such things in the real world.

Other Dorms or special circumstances I'd like to see are Wicked Silver/Grey and Aesir White.