As much as I do love the idea of leveling up followers and having them be more unique, I’m not sure I think it’s a good idea to convert all thralls to this new follower system.
The problem as I see it, is that it renders T1-T3 thralls absolute useless garbage. No one will ever bother with converting any thrall under T4 if they’re going to have to put in a significant time investment in training them up to level 20 afterwards, and end up with a follower that’s inferior to a leveled up T4 thrall.
My suggestion is this: Only T4 thralls should be able to level.
Leave T1-T3 thralls more or less as-is, with static attributes. Then instead of the general 55 follower limit that has been stated, instead allow a person 50 “thralls”, which would be the T1-T3 followers, and 10 “companions”, being the T4 thralls who can level up.
This way, garrisoning your base is not a grueling, tedious process of leveling up potentially 100 followers, you only have a smaller core of “companion” thralls which actually have levels and personality, and the time investment requirement is less onerous to max them out. The companion thralls serve as your bodyguards and such, guards of critical areas, and adventuring companions.
At the same time, you still have a larger number of generic foot soldier thralls who you can place on guard duty all around your base, so if you’re building a huge castle, it doesn’t look silly with barely anyone manning the walls.
I’m thinking default to a clan maxing out at 80 thralls and 20 companions, or something like that. So a 4:1 ratio or so.
Naturally server owners should have settings to adjust companion and thrall limits separately, if something like this were to be implemented.
I really do think there needs to be a distinction drawn between your relatively generic soldier thralls and your lieutenants and such with more personality though.