So, for Arena your progression is to Gladiator via CR. In BGB/RBG it is to GM/HWL via CR. The Achievements are already reflecting this.
Why not remove CR from BGB? Make a new metric that more accurately assesses your progression? Make it operate like Honor, but only Honor gained in this mode is added to your progression, or Rank. Keep the ilvl requirement to enter. Keep the seasonal format, just like Arena. Maybe add some seasonal flare to the top ranks. Maintain that this mode is an end-game PvP mode. PvE has a few different modes for progression, M+ and/or Raids. Why can’t we have different end-game PvP variants?
The Honor
In past expansions you were awarded honor not just for winning, but for performance. If criteria can be made to determine performance in support of the objective and team, then it can be used to earn more honor, even in a loss. Such as, Damage/healing done to FC, done while in proximity to a node or cart, support provided to the team in the form of Utility, etc. The “honor” in this mode would likely need to be inflated for ranks.
Ranking Up
I doubt players would be wild about waiting for a weekly reset to gain rank. So, make the ranks attainable in real time… with diminishing returns. The lower rank you are, the more ranks you can earn before DR kicks in. As an example, 4 ranks before DR in the early ranks, 3 in the middle and 2-1 at the top. Blizzard would have to determine the sweet spots here.
Rewards
Currently, ranks are tied to Achievements which are tied to CR. For this mode, the achievements would be tied to the ranks. I think the elite rewards would have to be at the higher ranks, depending on the pace set for ranking up. Starting out, I would say keep the rewards as they are. Ultimately, different variations of the elite appearances specific to this mode would be the best.
PvP Gearing
Starting out it could probably stay as is. Earning a certain amount of conquest per win or loss (if performance is taken in to account). Ideally, the ranks would allow us to buy new gear. Gear that would give bonuses in this mode. Still PvP gear with the boosted ilvls in PvP content, but set bonuses to aid in the 10-15 ( or greater) player environment. This would probably take the most time. Making new sets, determining set bonuses, etc. But it would add another desperately needed layer to PvP as a whole.
Maps and Player Count
So, the current rotation for BGB is… a little bit underwhelming. How about introducing a weekly rotation? Have three to four 10-15 player BGs. Keep the player count as is. Additionally, keep a wild card Epic BG in the rotation that only queues up when enough participation is reached.
Basically, this would be a natural progression from unrated BGs. Which, honestly could just replace unrated BGs altogether. This gets rid of the MMR black hole and provides progression with rewards.
Blizzard, you have three different versions of the game live, right now. A PvP renaissance is long overdue.