So I read Chris Evans wants to have a small role in Thor 2. I thought, what if he is present during Loki's trial? A witness who testifies against him.
This link is to an enormous and fascinating article on the institution of the Althing in Iceland in the Viking Era. The Althing was the annual legislative gathering, at which court cases also were heard:
http://www.hurstwic.org/history/articles/society/text/laws.htm
I'll highlight a couple of points relevant to this discussion of Loki's trial and the possibility of Steve Rogers serving as a witness, assuming they draw from Viking tradition to depict such a trial (instead of, for example, simply having Odin render judgement and execute sentence).
"The courts were not at all like what one thinks of as a modern western court of law. There were no public prosecutors. All cases were private suits. Usually, cases were prosecuted by someone close to the injured party, such as a family member, or a goði [a leader]. But an individual could bring action in a case even if he were not involved in the case." [And there was a panel of 36 judges overseeing the case]
"The magnitude of the punishment of full outlawry [the maximum punishment] should not be underestimated. Not only was there the psychological terror of loneliness due to exclusion from all social contacts, there was also the very real threat of violence and death from unrelated third parties who sought to increase their own prestige by killing an outlaw." [which was allowed without penalty]
It seems you could have Selvig testify against Loki. He certainly was injured, and is already in the movie.
Also note that they are supposed to shoot on location in Iceland. So they possibly could use the very valley in which that the Viking-age Icelandic Althing met as a location if they did an Althing in the movie.
My opinion is that Odin will render judgement from his throne room (and not bother with following Viking tradition for this plot point), and the punishment is likely to be exile from Asgardian society, but confinement to Asgard. (People have mentioned an Isle of Silence) Thor will probably provide testimony. It's too bad that the ravens didn't make it down to earth for TA; they too could have provided testimony and that would have been a nice touch. Unless they get Jane & Co. to Asgard immediately after the events of The Avengers (which I doubt), Selvig will not provide testimony.
If Captain America shows up in T:TDW, it will be in a stinger scene during the credits.