I also don't find that asking what the relationship wants is very helpful, at least not at the higher levels. (This may be where the "relationship as character" advice falls short.) I definitely don't think the answer to that has much to do with Domain, and while it might be related to Concern, it could be misleading.
"What the relationship wants that is problematic for or challenging to the relationship" does connect pretty closely to the RS Problem, I think.
RS Domain thoughts
I think the best way to think of RS Domain is, they relate to each other through (Domain). These relations affect the relationship meaningfully by bringing them closer together or farther apart. I think it's inaccurate, or at least confusing, to consider the Domain as always problematic. In a relationship, "source of conflict" can mean "source of challenge" or "source of change".
So for an RS in Psychology, for example, manipulating each other might push them farther apart. But manipulation might also draw them closer together, say as one tries to convince the other to strive for something, or to help.
I feel like a good way to look at the RS Concern is, it's the the thing that really challenges and tempers the relationship. It's the fire they have to walk through. It might burn them. But if they make it through to the other side together, the relationship will be all the stronger.
So the "fire" in an Obtaining relationship is the getting together, the losing each other, the asserting control.
The "fire" in a Becoming relationship is what they are becoming, what they might turn into.
In a Learning relationship it's the process of learning why they can't be together, or why they can; learning how much they hate or love each other; teaching each other.