Gonna try another one.
I think Grass/Dragon mSceptile was a shitty idea and done only for the sake of coolness. Likewise I think most people's "" unique "" suggestions for mega Whatever with an additional Dragon or Fairy typing are also shitty ideas done for the sake of coolness.
See: Dunsparce, Milotic, Rapidash, Mismagius, Serperior, pretty much every mono type without a mega on every Pokémon discussion board ever.
Charizard gets a pass because it was obvious really and Ampharos was a funny pun.
Well Mega Evolution is often times running on "rule of cool".
Dragon and Fairy do seem like those "special" types which everyone wants to add onto a Pokemon now but of course if they do that they'll make the types no longer special (heck, how glad were players when they saw none of Gen VI Legendaries were dragons?). I'm sure GF knows this so they're going to be selective who gets it, though that said who they think should get it probably isn't going to match what most people think will get it.
Taking your list for example. Dunspace, Rapidash, Mismagius, and Serperior are out of the question. I know there are justification for some, but Dunsparce is more heavily based on a Tsuchinoko than a Mesoamerican winged serpent (so I'm seeing more Normal/Ground, better hope it has something that can rival Huge Power in that case...), they'd have to go out of their way to design Rapidash and Mismagius into something that would be Fairy-type (and if Mega Absol isn't considered a Fairy-type I don't know what they'd have to do to make those two qualify) and people only want Dragon-type Serperior because they think it'll keep/get its Hidden Ability Contrary as its Mega Ability and somehow be able to get Draco Meteor. :P Milotic MAYBE has a shot at Fairy since they go on and on about its elegance and beauty, but if Gyarados didn't get Dragon I have my skepticism about Milotic getting Fairy.