As Anthony said, you really shouldn't create a group higher than superadmin for the sole purpose that it's expected as root. You won't break anything, per se, but superadmins are expected to have the highest permission in plugins not supporting ULX. So, any call to IsSuperAdmin() expecting to only match root users may have unintended results. You can create a rank higher than superadmin if you'd like, but it's recommended you instead just use whatever other addons you have available to you to change the appearance of rank names instead of creating a group as a workaround, especially if you're going to repurpose the superadmin role and give it to people you don't trust with absolute control over non-UL(X|ib)-integrated addons.