You could make it an unlockable after one finishes the game once. And you can lock down the name, portrait and voice.
Or at least make the first five mercs free. I mean would not many mercs pay is to get mythic powers for themselves?
The problem is game gets boring on replays, ending up with same meta builds for each companion over and over.
And there are no mods on consoles.
Oh, and maybe more people would buy the next dlc if they could apply the new classes on companions.
Tortured crusader Trever, Ghost Rider lich companion..
For PC, it's easily solved by a mod. I can see how a certain % of players on console would like to make use of it, but sadly there are many way more prioritized things to update on console to even consider it.