Just because I am thinking about it...
One could incorporate a "two under" approach with transfers into a THREE under. For instance, suppose that partner opens 2NT. You might play that a 3S response is a relay to 3NT, as the means of getting to 3NT. Then, 3NT would be available as a THREE-under call to show one or both majors, game-only:
2NT-P-3NT = one or both majors, game only.
If Opener prefers hearts, he bids 4H. In that instance, Responder would play spade contracts, unfortunately.
If Opener prefers spades (or equal preference), he bids 4C, after which Responder transfers to the suit we agree to play.
Using that approach might then allow Responder to use four-level calls (4C, 4D, 4H, and 4S) for other meanings, such as perhaps Texas showing 5-5 with that major and five or the opposite color minor, 4C as diamonds and hearts, and 4S as blacks, all slammish.
Again, this is not necessarily a matter of suggesting a good idea. It is a matter of brainstorming methods.
A three-under with transfers to show both minors would be a heart call, which is perhaps rather rare to imagine. A three-under for spades and clubs would be a club call; a three-under for reds would be a spade call. These might make sense in the context of a cue or double of the opponent's suit.