Here's a thought. Not sure how to crystalize this into practice.
Every so often, you see the hand where two possible strains offer reasonable trumps. For instance, maybe you find out that you have a minor fit and a major fit. Maybe 1S-P-2C-P-3C-P-3S as the start.
In this situation, a recurring theme is that of deciding ultimate strain in a potential slam. For example, if Responder has Kxx in spades, and Opener has Axxxx, perhaps a spade can be pitched from the short side in a club contract, allowing the spade suit to be played for no losers, if the contract is 6C. Or, maybe 6C simply reduces the risk from a 4-1 spade stack. What's worse is if we only have the spade ace and are missing both King and Queen, where 6C might be able to handle that.
Alternatively, however, 6C might not be as good for the same reason, and maybe a club pitch or two might help 6S to make.
The key to these situations is in the Queens, primarily, and secondarily in the Kings. So, what sometimes happens is that you use RKCB in spades, find out perhaps the bad news, and then bid 6C (and hope that partner takes this as a placement of contract and not a grand try).
It seems to me that there might be an alternative to consider.
Let's take the auction out a bit, in general terms. 1S-P-2C-P-3C-P-3S start. Cues and seriousness and the like end us at the point of, say, someone bidding 4H as Last Train or as a cue. Whatever. Now is the witching hour.
Suppose, also, that something about the "cues and seriousness and the like" suggests or proves that Exclusion RKCB is not an option. So, at this point, you might end up in a situation with an "ask or answer" structure.
What is that, you ask? Well, simply put, 4NT is RKCB, but anything above 4NT is an answer to an "assumed 4NT" from partner. Hence, if partner bids 4H Last Train (spades agreed), I would bid 4NT to ask questions, or I could bid 5H to instead answer, "Two without." I can ASK or ANSWER.
So, why "ask or answer?" I mean, which do you elect to do? I typically, in these situations (where partner and I have agreed to do this) answer with primes but ask with body.
There might be a slightly better way to handle this in the two-fits scenario. Or, a more precise rule.
If I am looking at the spade Queen, I am not as concerned about the "which strain" question. If I am not looking at the spade Queen, though, I can ask and place the contract in clubs if I have the club Queen. But, what if I have neither (and cuebidding has not answered this question, yet)?
Or, what if some Queen is unknown, but important? In some auctions, for instance, the spade queen might be known, but not the club Queen, because no cue was available to show that card. Finding that card later might be quite a task.
Here, then, is the thought. One might decide that you ask with the key unshown key Queen but answer without the key Queen. The side-suit, alternative-strain Queen is the default "key Queen." The part about asking with the key Queen is tactical, as captaincy is with the person looking at the key Queen. The part about answering without the key Queen is definitional, as captaincy is transferred to the person who may or may not be looking at the key Queen but now knows the answer as to the partnership holding by virtue of the definition.
I am still not sure how this would develop as a theory, as far as designation of the key Queen, handling a two-queens scenario, and the like. But, I think there is something to think about here.