Probably the next one would be "Mythical".
I like the sound of it. Ancient after thinking does kinda make the account sound old not special. Mythical has kinda a more magical sounding.
I can only wonder what activity for it would be. You would have to have it very high to make it so special. I like the random factor to, so random just so high very few would ever get it. I think it would be fun if only a handful ever got it the activity level was so high.
They should just double with every rank in my opinion. Makes it easier and doubling each time makes the ranks much difficult to achieve whilst keeping some standard/order. Legendary should become 960 and then Mythical 1920.
I like your thinking on this. That range is pretty good. I still would like it to be random as it adds additional factor that makes it even more special.
But if you did random with 1920 - 2XXX (2000 and something) I would like it. I just want this one to be one where when someone sees it they are impressed with the person getting it.
Also giving them a nice deign on the coin's beside name. I don't know how to make it super special. But it would have to be something worthy of the rank.