Well, we already have “gold” thralls in game now… T4. So let T1 & T2 thralls capable of leveling up to T3. Let T3 thralls know all the basic gear and recipes to craft, but still need T4 for the highest quality and even “special” recipes known only to T4 thralls.
With all due respect to the idea, I feel it adds an unnecessary layer of complexity to a system that’s already well established. Even if there were progression from T1 to T4, chances are players would find a T4 of the role they’re improving before even finishing the earlier tiers. And that’s without considering the technical barrier Tephra pointed out.
If T4s didn’t exist, I’d find the idea fantastic. But since we already have the best in their roles, recognized by their unique names, I think that fulfills the purpose well.
I see this as similar to the case of the T4 golden bar, which had to be adjusted to resemble the original. Features like the T4 bar, the thrall system, and others are part of Conan’s identity. These defining elements should only be changed if there’s a truly special reason.
That said, mods are there precisely for this kind of experimentation. Who knows, maybe someone will create something similar, the community will enjoy it, and it could eventually make its way into the game. It wouldn’t be the first time. 
I agree with you to an extent, the gold-named T4 thralls already fill a great role as the “best of the best” and are instantly recognizable. That said, I don’t fully understand why people shoot down optional systems that others don’t have to engage with. This kind of progression wouldn’t replace the current T4 system. It would just give players who want more to do with their thralls an alternative path, instead of purely farming and praying to RNG for that perfect named one.
Some of us enjoy the idea of taking a favorite thrall and gradually improving/perfecting them over time, rather than always chasing the next easy gold bar drop. If you prefer the current system and just want to grab your T4 and move on, that’s totally fine, this wouldn’t affect you at all.
I still think adding a proper crafting perk tree (or specialization system) would be awesome. Things like a one-time legendary repair perk, unique secondary bonuses, or small playstyle edges that let different crafters feel distinct. Games are at their best when players can carve out their own little identity and strengths. Just my take.
This would be like me saying I don’t like something others enjoy and wanting it removed simply because it’s not my cup of tea. More ways to play, more paths to forge, and more options for players is what truly makes sandbox games enjoyable, it brings replayability and hours upon hours of meaningful things to engage with.
You should «feed» high-level things to them to upgrade them to high tier. I’d make it like fishing in WoW: the higher your fishing level, the less experience you get from catching entry-level fish, until you get no experience at all when the fish are too low-level for you. But if you try to catch high-level fish at a low fishing level, you won’t get anything: neither fish nor experience. It’s a good idea for training thralls: they can dismantle everything; high tier thralls get most materials but no experience dismantling low tier gear; low tier thralls get almost no materials or experience dismantling high tier gear (they spoil it because of lack of skills and can’t get any knowledge because they have no basics for it). All of them gain the most experience by dismantling gear of their tier+1.
But at no point was it a criticism; it’s simply an opinion different from yours.
The main reason I’m not on board with it is because it’s a lot more work for already over tasked developers than adding in a smaller, discreet system that increases the crafting tier/skills of low tier crafters. If you’re going to propose an idea you need to make it realistic, otherwise you might as well be writing fan fic.
lot of other stuff to work on for the small dev team.
just go find a thrall. most of the pve players enjoy thrall hunting. so i dunno.
nopers
Great idea I think, I’m personally really tired of seeing everyone use the same thralls and this system will allow players to create their own unique crafters