With housing on the way, and with it dyeable furniture, Midnight would be the perfect time to introduce a similar dye system to transmog.
But I think perhaps the first iteration should be somewhat limited in scope to not be so burdensome on the developers and tied to a collection meta achievement.
Achievement Proposal (using The War within: Season 2 as an example):
The War Within Season 2: Ultimate Fashionista(, Evoker)
Requirements:
- Can You Please Spell “Gobanna?” (Complete an armor set from Liberation of Undermine) > Fashionista: Undermined (Collect all 4 armor sets from Liberation of Undermine)
- Prized Guise (Complete a War Within Season 2 PvP armor set)
- Dressed to the Mines (this is permanently obtainable by completing the Mythic difficulty of the raid)
Reward: (Evoker) Undermine Dye Kit
This reward would let you customize the specific tier set by mixing and matching any of the colors used in any of the known appearances (The Elite set isn’t required, but can be used in this system).
The intention for this achievement would be to complete all permanently available versions of the armor set, or needing to collect all but one (All PvE + one PvP or all PvP + 3 PvE). This could be specific to the class or warband wide, depending on Blizzard’s preferences.
(The second option makes PvPers have to interact with minimal PvE content, as all colors are possible to get via PvP gear into catalyst + upgrading with Runed crests only.)
This would allow greater customization of the tier sets, while requiring hopefully little retooling, if the sets are recolored in such a way that the color-ways are separated. Would also still restrict color possibilities to what is intended for the season. Would add more prestige/collection content for hopefully little fuss on the backend.
This system could be debuted in Midnight, but also include all sets from TWW, with legacy set achievements being added periodically (which is why I think the Elite set should not be a requirement).