You have to have an Arcane Art I think. Without getting inmate spells that compliment each other, it's impossibe to reach morning star rank. I think that means they have to be capable of an arcane art if they are that complementary.
When leylin joins oroborous clan, they show him an arcane art recording and inform him about how its possible to not have an arcane art.
I think it was mention that some fail arts would somehow let you advance without giving you an innate spell at some ranks
Or that the spells are complementary enough to advance, but not merge into an arcane art
it was definitely mentioned that there are some "failures" who manage to reach rank 4 but without arcane art.
oh, it might also be because they brokethrough using the "forcibly learn a spell of the next level" method instead of high grade meditation technique
Oh, and various species from other worlds breakthrough with their body so they do not have arcane arts. so maybe its possible too for magi who focus on body cultivation?
Some meditation techniques are also defective or incomplete.
Supposedly originally meditation techniques didn't have arcane arts, but those were developed by generations over tens of thousands of years in the past, specifically engineered into the meditation techniques
Oh IIRC it was also mentioned that if you do have slightly incompatible spells, then instead of exploding you could still advance, but you would be weaker than others as you need to constantly spend a portion of your power holding yourself together / healing the damage of the incompatibility. I am also amazed someone like that was even able to get to rank 4 at all, how could this even be? I think the author of WMW just didn't really think it through.
Even though we were told about rank 4s without arcane arts, the only ones we ever saw were creatures that advanced with their bodies.
I think power creep was involved in the whole thing too. author needed new scrubs that are not too scrubby. so suddenly there are more higher ranks