3. Adam Strange
Adam Strange is basically a more science fiction version of the Princess of Mars story by Edgar Rice Burroughs. Adam is periodically transported to a planet called Rann where he meets a beautiful woman and protects the planet for various threats. He has no powers other than his intellect, but he sports a jet-pack and various ray guns. Adam Strange could easily become DC‘s version of Star-Lord and inspire all sorts of space-faring stories for the league to take part in.
One particular storyline featured Adam kidnapping the Justice League forcing them into slavery as all part of an elaborate plan to seek their aid in freeing his people and planet from an oppressive force. Adam would bring a more human element to the team. He would also fill the badly needed role of scientist for the league. I know Batman is smart, but a true scientific genius on the team would make things a lot easier. In comparison to Marvel, he could be a combination of Tony Stark and Peter Quill.