I respectively disagree with you here, not every Pokemon needs to belong to an evolutionary line. Mega Evolution allows those Pokemon that don't evolve like Pinsir, Absol and Mawile to become the best version of themselves, without being restricted to have to just evolve and turn into an entire different Pokemon, which would leave them in obscurity. No one was using Mawile, and if Mawile got a regular evolution, even if it resembled Mega Mawile in some way, no one would still use Mawile as it literally just got replaced by Mawmawile who you acquire by spinning your 3DS like a top and giving 5000 pokedollars to the Mew under the truck. Mega Mawile, like Mega Absol, Mega Pinsir, and every other Mega is just the same Pokemon as before, but the best version of them that leaves players the option to use the mega stone or not. Megas breathed new life into Pokemon that people had forgotten, and adding a brand new evolution to those lines would still leave those Pokemon in obscurity, forever be forgotten as a not-fully evolved Pokemon.
I'm glad that you are beginning to understand what the creators have always thought and that all of their Pokemon they created, are finished creatures regardless of if they get new evolutions or not in the end, and megas are a way to expand upon those finished creatures without ruining the base form.
I'm gonna disagree with a bit of this.
When Gen IV introduced Pokemon like Togekiss, Yanmega, Mamoswine, etc, I don't think it kept those Pokemon in obscurity - in fact, it pushed a few of them to the forefront
cough Eviolite Murkrow, Dusclops, and Porygon2
cough. I'd love to have seen at least Absol, Mawile, and Sableye get regular evolutions before Megas. (God, especially Sableye. I'm still salty over it's horrid Mega design).
On that note, I want Skarmory to get an evolution, then give that evo a Mega. Because Eviolite will always be one of my fave items.
Regular Megas I'd like to see at some point aside from the obvious Flygon, Butterfree, Solrock, etc,include:
Mega Sunflora (or Bellossom honestly)
Grass/Fire
Chlorophyll
Mega Ludicolo
Grass/Water
Some kind of Own Tempo/Magic Bounce combination
(I just want a Ludicolo that Miror B. would be proud of)
Mega Cincinno
Fairy or Normal/Fairy
Celebrate - only viable in non-Singles. If this Pokemon defeats another, it boosts the other Pokemon on it's team's Attack. (That's worded really weird oops. It's like Moxie but it raises your other Pokemon's attack instead of the one who has the ability. And yeah I totally ripped off the Ability name from Conquest)