How do you think they should be portrayed is in some ways more important then who is cast as who. I've been giving this some thought and even though I have not read the script (has it even been finalized?) and how the director can often change things I've been thinking the following.
Aesir-The Aesir in general should be like Vikings in the cultural mind. Maybe a bit more noble, but assuming this is before the Aesir changed into the more Arthurian knights mode or the very least that may be something only the upper-class impress the average Aesir should like Vikings. They enjoy life in all ways, are in a ways easily angered, like to boast, and a full of confidence because they are gods.
Thor-Since this is before Thor is Mr. Noble I picture him as being like the Aesir described above, but even more so. Women want to be him and men want to be him. He is a god among gods and he knows it. His boasting and arrogance are to a point that if you are around him enough even he annoys you. His boasting goes beyond what is even acceptable to their society. He is easily provoked and may even pick fights to show how tough he is and impress people. He is a great guy to have in a fight or at a party, but if you are around him too long he gets annoying. He is the type to butt his way in where he is not invited and steal everyone's girl. His overall character at this point is a combination of Conan and a bully. If I were to say he is close to a Shakespearean character I would not know which one because the plays of Shakespeare I am familiar with do not have someone like him. The only other closest character I can think of is Achilles from the Illiad and not the film Troy.
Sif-Sif would be portrayed as Thor's love interest and as a friend, but she would be one of those that know the real Thor and that would strain the relationship. Since he is so full of himself for his own good she can at times barely stand him. Despite that, she would stand by Thor if needed.
Odin-Odin of course would be important to the story. I picture him as like the traditional Odin, sitting on a throne with a long beard, but a grim continence. From just looking at him you can tell he is grim, but also powerful and deadly. He also looks as if he has the weight of the universe on his shoulders with his current major problem being Thor. Thor has become somewhat of an embarrassment to Odin due to his huge ego and complaints of Thor always picking fights. Odin is one of the few beings who still commands Thor's respect at least to his face and that Thor will obey. At the same time, Thor considers and over-bearing meddler who puts his nose where it doesn't belong and it is only a matter of time before things between the two come to a head. If based on a Shakespearen character I imagine Odin closest to Prospero from The Tempest with more a of a warrior look.
Balder-I imagine Balder is going to be in it. I picture Balder as being more of a lover than a fighter with the soul of a poet. He would be one of the few beings Thor would actually show respect for. Balder is one of the few that can get Thor to calm down and actually stand his company. I picture him a lot like Romeo.
Loki-Loki is of course the villain of the piece with two-sides. To the Aesir he presents the face of a harmless being...almost a sniveling servile coward or sycophant. He would already been known as the god of mischief, but would not be considered a deadly threat capable of only petty magics. More of an annoyance the other Aesir look down upon due to his giant's blood and only tolerated due to Odin's orders.
The other face would be the true Loki. The dangerous, powerful sorcerer who secretly plots the downfall of Asgard. He would be the viper that constantly whispers in everyone's ear trying to set father against son and destroy relationships all the while appearing to be perfectly helpful and harmless. I picture him like Grim Wormtounge from LOTRs or Iago from the Shakespeare play Othello. Only Odin would be the one to suspect Loki for what he really was.
Loki ironically would be the one thing about Thor that is redeemable. Thor for all his posturing would have a close relationship with Loki and refuse to tolerant anyone insulting him. This would add a dimension to the relationship so that Loki would have a legitimate grievance against the other Aesir the one person who he would focus his hatred on is the one person who does not deserve it. This trust Thor has in Loki would also make it all the more painful when at the end.
There is also the matter of costumes and set design which can have just as much effect as anything else. Afterall, how would you think of Thor if he is dressed up like a court jester?
Costumes-The early Jack Kirby outfights fight work or the current Coipel ones would be best. You would not want to change too much from the comics, but then some of the more outrageous ones espically the helmets I think would not translate well into the films. You would have to do something close enough to the public ideas for the general public to take it seriously, but at the same time close enough to comics to please the fans and make it recognizable to him.
Asgard-I know a lot of people like the space-age Asgard, but considering this sounds like it is set in the past it would again probable be best to go close to something like Gondor or Rohan from LOTRs. Some combination of the two. At the same time make it look shiny enough so it is very rich.
As far as costumes go, a more practical approach would be either the Coipel costumes or, for Thor mostly, the 'Ages of Thunder' outfit would work the best. Not too far from the classic and it has some of the most defining/iconic parts of his aesthetic design: the winged helmet, the red cape and the four chest 'circles'. (Notice how any Marvel*Thor design has at least those 3 features?)
Which brings me to my next point: The important part is just having the identifiable basics of each character. Sif's got her red-white theme and her winged headdress and Loki is all green-gold with horns, to name a couple examples. Integrate those into a practical-for-cinema costumes (I mean, the Hulk didn't have purple pants in his movie from what I could see) and you're as good as gold.