Maybe don't change too much from Legendary, just add + and add more gold from 5 to 6 like the picture below.
Legendary/ (pronounce: "Legendary Slash") sounds much cooler.
These might "sound" cooler, but what about adding a BTC symbol instead of anything random.
Legendary BTC or BTC Legendary
While I remain opposed to another rank, I would argue (if there really is a new rank) for a clearly visible difference from Legendary status. Like for example:
Epic
Elite
Veteran
Adding the ranks you mentioned above would be a fresh journey for the Legendary members which in turn make them hustle for more merits in the forum, but it'll demoralize some lower ranked members knowing that the journey is yet too far to get to elite. I think the best method is to change the color of the last coins on the legendary member's account who manages to hit the desired merit and activity count for a new rank. Instead of changing names that'll be a better way to distinguish between different level of legendary ranks. On the other hand, the bitcoin sign you suggested, has been set aside for the core bitcoin developers and that will cause some complication when trying to determine who is indeed a developer, or worked with Satoshi.