Or the Mighty Avengers, like we're getting The Amazing Spider-Man or had The Incredible Hulk.
That's not to do with Copyright. As far as copyright of a film/ work title is concerned, I don't think it can be copyrighted. That's why several books, songs or even films can have the same name as a previous one - eg two films with the name "Black Rain" were released in the same year (1989), almost at the same time, both with strong Japanese themes.
If there's a legal issue, it's to do with Trademarks and/or Passing Off with regard to brand names.
So in the case of the animated movie "The Iron Giant" which was adapted from the novel "The Iron Man" by Ted Hughes (rechristened "The Iron Giant" in the US), if Marvel's Iron Man weren't already a trademarked character, then there wouldn't have been any issue with having two films of the same/ simlar name - ie The Iron Man and Iron Man.
With the 60s series "The Avengers", that is probably a trademarked franchise over here in the UK. The thing is however, that two things with the same name can be trademarked in different fields - eg Iron Man as a comic character and Ironman as the triathlon event.
Now both the 60s series "The Avengers" and Marvel's "The Avengers" are in the same entertainment/ film field, but the question is whether there is some kind of sub division within that field that would allow both to co-exist legally - eg the 60s series "The Avengers" might only apply to a spy genre whereas Marvel's The Avengers is a superhero genre. Would there be confusion between the two? Probably not, but I don't know how broad the scope of the 60s Avengers tv series trademark is such that it excludes Marvel's The Avengers from using that name in the UK.