Itll be interesting to see how far into the "super hero/villain" realm theyll go with DD. Like will they keep it more of a grounded vigilante type deal or have the fantastic sci fi stuff come in.
Based on the trailers and talk from cast and crew, it seems like a safe assumption to say they appear to be going more towards street criminals than sci-fi or super villain. I mean, that seems to fit what they have said about Daredevil in particular and perhaps even this corner of the MCU as a whole. It feels as though the identity of The Defenders wants to be less about the sort of superstar style heroics that shake the world and instead focus on homegrown everyday heroes taking on every day evils. Compared to the likes of Tony Stark and Steve Rogers, what Matt is doing makes him something of a faceless nobody, so it's fitting that the criminals he goes after are much the same. They aren't special in the way the Red Skull or even someone like Donny Gill from Agents Of SHIELD are - they aren't on any index, they aren't part of some plot for world domination, and stopping them doesn't land you at a senate hearing in front of a hundred reporters. At most it earns some gratitude from the old lady they just tried to rob, but more likely it's rather thankless, necessary work.
I get the feeling that Daredevil and the rest of the Defenders are going to be portrayed as something closer to hardworking, honest cops than whatever you would compare Thor to. I just mean... There are different levels of "bad" and things we need saving from. DD is there to clean up the streets, keep us safe from the threats that have always been there long before any super soldier serum started us down the path of spandex and capes. If someone is walking down the middle of Broadway with axes for hands and laser beams as eyes, someone is going to call f&#(ing Iron Man. If someone is holding up a grocery store or exploiting the local businesses for "protection", Tony is probably not picking up his phone for that one.
Of course there is something to be said for some old fashion spandex style vaudevillian. As a big fan of Iron Man 3 I actually love the route they went with The Mandarin/Trevor, effectively giving us that handlebar mustache twirling "I must be a plain dealing villain" character and then making the whole thing, well, an actual
character. I'd LOVE to see what those kind of guys look to through the eyes of your more grounded, street level hero (same reason I'm hoping to have some major comic booky bad guy pop up in Cage's series to be honest - I just think his reaction to that kind of crazy would be hilarious).
So while I'm not dismissing the idea of any true super villain style characters outright, I would guess they are sticking more with lowlife offenders, organized crime, and the sort of scum that might be considered to petty to get The Avengers involved. At the same time, I'm betting those kind of criminals effect far more lives than Loki ever did. They just don't get you parades and merchandise the way saving New York from an alien attack might.