My opinion is that couple is more or less the straight version of yaoi and yuri. So throwing them in there would just bog it down without even adding that much information to the stuff under yaoi and yuri.
Well then, what about girl-with-girl and boy-with-boy images that are not romantic? The homoerotic equivalents of the images in -rating:s -solo -yuri -yaoi -couple (that search might not work, I just put it there as a definition)? For example, right now there are seven pages of yuri rape.
It's fine by me to turn yuri and yaoi into couple equivalents (that's what some commenters seem to expect), but we'll need superset tags that include the other stuff too. How about using gay and lesbian (unless we want both of those for the LGBT identities, including solo pictures)?
IMHO, however, making yuri, yaoi, bisexual, futa_with_* and the implicit hetero orthogonal both to ratings and to the couple tag would produce simpler searches and less confusing tag definitions. I'd also like to promote romance as an alternative to couple with a less ambiguous name. We could even redefine couple (or maybe duo) to include platonic couples, any two characters that the image presents as (close) companions.
Yes, couple is the heterosexual yuri/yaoi, I think. It has an unfortunate name and I wouldn't mind changing that (not like homosexual couples aren't couples, after all). But I don't think the solution is to break the tag's intended meaning. ;)
couple includes any picture that explicitly identifies two people as a couple. Kissing or just holding hands, depending on the picture, is enough.