we have amazing and interesting companions (and Shadowheart) ...
AHAHAHAHAHA!
I agree that it is problematic that many origin characters are tied to certain classes. For me, the issue is not necessarily tied to the companions but rather that the origin characters themselves are tied to classes if you play them as well (which I am just going to assume they are - otherwise I can't wait to ruin the game by playing a paladin SH or barbarian Gale
).
However, while I've argued many times that the lack of class fluidity in BG3 is a very good argument to not keep the party locked at 4 characters, I wouldn't argue to MAKE companions/origins class fluid as much as I would argue for ... Well, bigger party size. At least 5.
The thing is, in DOS universe I never felt like your class was that important. In any aspect, at all. You were all sorcerers regardless, and the "root" of your power was the same regardless of how it manifested and how it was channeled. In Forgotten Realms however, classes do not at all have the same origins of power which makes origin characters with open class choices feel... Weird at best, world-conflicting at worst.
Anyways - you specify that it should be something close to the original class, such as fighter/barbarian or paladin/cleric. I am going to openly admit that I am not well-versed enough to say whenever this would be a huge conflict (although my guess on the paladin/cleric one would be "probably"). Personally, I would argue for opening multi-classing entirely (at least for the main PC). And if not entirely for companions, then at least do it as PoE:D did it and have the options for 2 or 3 multi-class combinations for each companion.
I'll make some examples to illustrate my point (do note that the examples are not necessarily proper suggestions and the names are completely made up in the moment, so apologizes for any confusion): Shadowheart's base class is cleric. But, you could also choose to play her as a Nightwhisperer (cleric + warlock - think of Cipher in PoE), Shadow Bender (cleric + rogue) or Witch (cleric + druid).
... ... And, of course, if Larian intends on forcing each companion to have a singular class, then I would seriously hope that they'll at least make a companion out of each class. :|
