Eh, I can't see a whole Superman film with Metallo as the main villain. He's too one-dimensional of a character...
Superman's always one of my favorite superheroes but I tend to be ambivalent about his rogues gallery aside from Lex because of how alien (Darkseid, Brainiac) or goofy his villains can be (Toyman, Bizarro)... but MOS made me actually find Zod interesting, so I'm open to unexpected villains.
However, I
always thought Metallo was fascinating because of how grounded and human he
could be... in a world with Cyborg, he's incredibly plausible (heck, without Cyborg he's inevitable), and after S:TAS it's clear so many themes you could follow-up on. I was worried that Ultron would suck up all the air around the concept, but they basically ignored it all and reduced him into not much more than a beat-em-up boss with a time-limit, endless minions, and where you keep fighting upgraded versions. They didn't really explore the interesting themes....
For just the basic concept, we're talking about a human mind transcending into a man of steel... that artificial body and all its implications is at the heart of so much scifi (the Positronic Man, etc), philosophy (what does it mean to be human?), and modern internet angst (where we may exist as digital entities as opposed to corporeal beings). In S:TAS they explored hedonism and sensation, memory identity and morality, and mortality and immortality with just their "limited" version of Metallo. The transhumanism angle alone gives him the potential to be incredibly compelling.
If you broaden Metallo further, he can basically stand-in for the Avengers... if Metallo starts out as a human mercenary or military, that's Captain America, then per S:TAS if fighting in a massive hulking mecha-suit, that's Hulk, then he becomes Metallo, Iron Man, but if steps into technopathy or becoming a digital elemental... he's something more akin to a god, magic, or omni-something, Thor / Scarlet Witch / Vision.
Metallo's been everything from a shape-shifter who can disguise himself as anyone to a giant-skyscraper tall Godzilla-like monster. He's far from one-dimensional and has a lot of potential. Those sliding threat also justifies leaving the League out of it because Metallo can seem just manageable but actually not. Also, I don't think Metallo's characterization has ever been so consistent or cherished that anyone is going to throw a fit if he's used as mainly a template for more compelling concepts rather than just applied directly.
...um... basically, I'd be
really thrilled if Metallo was a villain, probably moreso than Brainiac.
[Plus, Metallo means Superman gets Kryptonian Anti-Radiation Armor... merchandising everybody, merchandising!]
