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.