I would honestly be in favor of retconning a bunch of legendaries out of existence. - That is, to say...they make evolutions and pre-evolutions cannon for some of them, make them breedable, etc. - Just spin it in a way that scientists thought there was only one in existence...but they found more (this is something that happens in real life, btw).
-Right off the bat, I'd do this with Cobalion, Terrakion, and Virizion. - I never thought these deserved to be legendary designs. (They don't look any more special than something like Luxray or Pryroar, IMO). You could easily give these first and second evolutions and make them ordinary (albeit a bit rare).
-Kubfu and Urshifu also have no right being associated with the "legendary" moniker in any way. - I actually like these Pokemon a LOT...but, how are these different from any number of other mammalian-based Pokemon? Being a two-stage Pokemon with an evolution should count it out of the discussion right from the start. - I didn't even know they were considered "legendaries" until I started making this post. Seriously, who gave this the green-light?
-Keldeo? Fun design...for a normal Pokemon. Give it an evolution or two. (This one is probably the biggest head-scratcher for me). The thing literally screams "I want to evolve into something better."...and the mere existence of Keldeo becomes crazier when you consider that Glastrier and Spectrier are a thing. - Seriously...why did they do this?
-Meloetta. Literally looks like one of the Gardevoir types. Another one that feels like it wants a pre-evo and post-evo design. Just doesn't offer anything that says "I'm one of a kind and special." Not a single thing...
-Zeraora is a Lucario clone. To me, it feels like this Pokemon was meant to get the same treatment as Lucario...kind of like when Marill first debuted in the US, and people called it Pikablue, and it had this kind of mythical status for a while. I could feel the same being true for Zeraora. Heck...when Lucario first showed up, people treated it kind of like a legendary as well. I'd argue that Lucario would have had more business being a legendary than most. But since he isn't...I don't think Zeraora should be either.
-Melmetal and Meltan...again with the weird pre-evo legendaries. I honestly don't think Meltan even needs to exist. - If Melmetal existed without the Meltan pre-evo, I'd be accepting of its legendary status...but I really don't like the trend of Legendaries with worthless pre-evos. Also, this is another one that I would compare to something like Gimmieghoul and Gholdengo. Why is one of these a legendary, and one is a normal Pokemon you can obtain? Both require collecting a large number of items to evolve a weak base Pokemon into a more powerful one. They are the same concept and delivery of said concept...but one is special? Why?
-Zarude is the same as Passimian and Oranguru. I'll not hear otherwise. All three of these need pre-evos and post-evos. They should all fall in the same category. Nothing special about Zarude that should place it in a league of its own.
-Enamorus. I have honestly not seen this Dragon Ball Z villain until this exact moment in time. I am laughing.