I would like to disagree with the opening statement, as I believe that the only approved concept that we couldn't make work is Choiced Choice Scarf, which is only because a Pokemon can't hold two items. I understand what you're trying to say about taking full advantage of Mega Evolution, but I think we have many ways in which we can do that. Even if the chosen concept isn't about Mega Evolution, during the process there will be discussions on how to make the most of the Mega with the chosen concept.Even if I'm not very into CAP concept discussion, I just wanted to say that The Patient Mega is almost the only way of realizing a Mega CAP, for three main reasons:
A change of typing upon Megaevolution would be a method to realize this concept, because it would give the CAP different weaknesses. I made a list of all the typing changes that modify totally the weaknesses of the Pokemon (I only added a type to single-type, tell me if I missed any combination)(if it isn't possible, I wrote some combinations of typing+ability to change the weaknesses or nullify them between "[...]"):
- We don't have yet a Pokemon that takes a real advantage of this characteristic of Megaevolution. There are only some examples, like Gyarados or Altaria, but none of the existing Megas focuses on this possibility offered by the Megaevolution;
- Making a Megaevolution that is only an improved version of the CAP, would make the existence of the base form useless, because it would mean that you wouldn't use that CAP if not with a mega stone;
- Making two different concept for the base form and the mega would mean that the base form can fulfill its role even without its mega stone, constituting a Pokemon on its own. You would have to decide to use the mega or not in base of what you need in your team.
- Grass (Bug, Fire, Flying, Poison, Ice) -> None [Grass/Steel+Flash Fire (Fighting)]
- Water (Electric, Grass) -> Water/Dragon (Dragon, Fairy)
- Fire (Rock, Ground, Water) -> None
- Electric (Ground) -> Electric/Flying (Rock, Ice), Electric/Bug (Rock, Fire), Electric/Grass (Fire, Ice, Poison)
- Flying (Rock, Ice, Electric) -> None [Flying/Steel+Volt Absorb/Lighting Rod/Motor Drive (Fire)]
- Fighting (Psychic, Fairy, Flying) -> Fighting/Steel (Fire, Ground, Fighting)
- Psychic (Bug, Dark, Ghost) -> None
- Ghost (Dark, Ghost) -> Ghost/Dark (Fairy)
- Poison (Psychic, Ground) -> None [Poison/Steel+Levitate (Fire), Poison/Dark+Levitate (None)]
- Ice (Fighting, Steel, Fire, Rock) -> None
- Bug (Flying, Fire, Rock) -> None [Bug/Steel+Flash Fire (None)]
- Rock (Fighting, Water, Ground, Steel, Grass) -> None
- Ground (Water, Ice, Grass) -> None [Ground/Water+Sap Sipper (None)]
- Steel (Fighting, Fire, Ground) -> None
- Dark (Fighting, Fairy, Bug) -> Dark/Poison (Ground)
- Normal (Fighting) -> Normal/Ghost (Dark), Normal/Psychic (Bug, Dark), Normal/Flying (Rock, Ice, Electric), Normal/Poison (Ground, Psychic), Normal/Bug (Flying, Rock, Fire)
- Dragon (Fairy, Ice, Dragon)-> Dragon/Steel (Fighting, Ground)
- Fairy (Steel, Poison) -> Fairy/Steel (Ground, Fire)
Electric and Normal are obviously the types that offer more possibility, because they have only one weakness.
EDIT: I bolded the rarest or not exisisting in OU type combinations.
EDIT 2: I discovered also (it may not be a new information for you) that adding 32 base points to a stat is almost equivalent to giving it 252 EVs (+1 point). This means that, for example, if we give to Mew 252 EVs in HP and 252 EVs in SpD and we make it evolve in a mega that has a stat spread like this: 68/132/100/100/68/132 (I know that decreasing HP upon Megaevolution is impossible, but it's only an example), this mega's stat spread would be almost the same as a 252 Atk / 252 Spe Mew. This means that 32 base points are sufficient to turn a supporter into a sweeper :D
As a side note, I like how you pointed out many type combinations that could give the Pokemon reasons to not immediately Mega Evolve!