Sure Theymos might make a new rank, some day.. especially as the number of merits continue to increase.
Theymos might do it, but there are other things should be considered, besides new rank (might be only one, above Legendary).
What to do with old-era, grandfather Legendary? There are Legendary users who make good contributions since January 2018, and earned hundreds or thousands of merits so far, but they are limited. In contrast, there are more significant Legendary users who have not earned not many merits, that discloses their post quality, over last one year and a half (since the day merit system began)
There are about 7% (111/1602) Legendary users whom earned more than 60 merits within last 6 months. It means part of more than 93% (active users) have issues with their post quality.
Legendary: 111 out of 1602 (>= 60 earned merits in last 6 months) --> 6,93%
Hero Member: 116 out of 1865 (>= 48 earned merits in last 6 months) --> 6,22%
Sr. Member: 82 out of 3321 (>= 42 earned merits in last 6 months) --> 2,47%
Full Member: 116 out of 6167 (>= 30 earned merits in last 6 months) --> 1,88%