I personally think that Heroines is more likely than couples since the last Two-pack of LEs did SO poorly. Merida/Elinor is still online and I don't think it's ever been marked as "sold out" once since it's debut. Not to mention Disney would be absolutely BONKERS to pass up a chance at a designer Merida, given her insane popularity, which even Disney underestimated when they first marketed her! Merida could NEVER be included in a "Designer Couples" line and I think THAT ALONE is why we can chalk Designer Couples up to just a rumor.
Let's not forget, the reason that villains REALLY happened was because Disney saw lots of people wanting villains dolls and making their own designs for them. If everything I've seen online (here, deviantart, tumblr, facebook, etc.) is ANY indication, Heroines is what is most wanted next. Could they do wedding or fairies? Yes, but I don't think the fanbase of the designer dolls is crying out quite as loudly for those lines. Not yet at least.
I'm really guessing Disney has learned it's lesson with the Designer Villains line's issues. Higher price + higher edition size + less popular characters = BAD FORMULA!
I think we're most likely looking at 4000-8000 edition sizes again and somewhere around 8 dolls (plus one, if you consider a D23 Silver Cinderella equivalent for this line). Not sure if they'll keep the $80 price tag or reduce prices could be anyone's guess. If I had to wager a guess at who they'd be, I'd probably pull girls from the R/C Disney Girls pins, or the Disney Birthstone pins first.
Merida - She is more a definition of "Heroine" than any other girls. And Disney would be crazy not to.
Esmeralda - Design possibilities are outstanding as well as a higher fanbase for a lesser-known character.
Megara - Same reasons as Esmeralda
Wendy - Popular and classic to this day. Could easily be marketed on journals, cups, shirts, etc.
Alice - Same reasons as Wendy. And Alice's 17" LE doll remains one of THE MOST sought after to this day.
Jane - I'm actually really including her because I find she still has a lot of representation in the parks (and she was featured for the birthstone pins). More so than Megara, for sure.
Tinkerbell - Crazy popular. Although it'd be the first time we get two characters who appear in the same film (Wendy and Tink), I just can't see the next like WITHOUT Tink.
Vanellope OR Lilo - the designs for each could be interesting. Vanellope's based on candy, Lilo based on a designer's take of a hula ensemble? I think if we could make the argument to stretch Vanellope out, we could make the same argument for Lilo. While Wreck-it Ralph is new and popular, Lilo and Stich has withstood the test of time.
I think Merida's potential alone would have made Disney go forward with a Heroines idea. All Designer merchandising has had one doll featured at the helm. Maleficent led the Designer Villains (her image was ALWAYS larger in the advertising) and Cinderella led the Designer Princesses. Merida could EASILY lead the heroines in the same way. I also think we're getting a couple of hints towards a Heroines line. Didn't Disneyland just do "Forgotten Friends" week for Limited Time Magic?? Esmeralda and Jane were apart of that. And Megara still has park representation (especially in the case R/C Girls and the Birthstone Pins).
I think we also have to take into consideration that Disney already has a PROVEN FORMULA in the marketing of singular designer dolls. In both the Designer Princesses and the Designer Villains, we saw the same box structure, the same bag around the box, the same journals, the same mugs, etc. If you throw a second doll in the mix (in the case of couples), then ALL of that would have to be changed. I also noticed a lot of people seem to have forgotten that there are also cosmetics marketed in the designer lines. How exactly does one create a nail polish color to represent a couple? Or an eyeshadow color for a Disney Prince? Just other things to consider.
Sorry for the long post, everyone! I just find it hard to believe Disney would choose Couples over Heroines, Fairies, Princes, or even Bridal. And I think from a marketing standpoint, knowing how Disney has run this line in the past (creating Designer Villains FOR the fans), Heroines or "Disney Femme Fatale" or "Disney Damsels" or whatever you wanna call it, makes more sense.