These are very rough sketches I banged out for a PBP I'm involved with. My goal was to do "realistic" takes on some of the classic JSA characters. So their costumes mostly aren't form-fitting, and are made of materials that make sense for the late 1940s, but they still have the look of the original characters. Some of them were pretty easy, really.
There wasn't much to work on with Mr. America. He wears a poet shirt, a cape, and a mask. So I just departed completely and gave him a suit and tie. The Americommando outfit is heavily inspired by the WWII Ultimate Captain America outfit from vol. 1 of the Ultimates. It'd be a pretty much standard uniform with a different cut and different colors - and on actual operations he probably wouldn't wear it too often, because then he'd be sniper-bait.
For Dr. Fate, I went with a long cassock instead of tights. He loses some of the Egyptian motif of his costume, but not all. The embroidery on the cape and cassock would be reminicent of the metalic parts of the classic costume. I'm thinking he might also wear gold demigauntlets, which would just look cool. One alteration I might make is to go with the half-mask version of the helm instead of the full facemask, but I think the full one looks cooler.
Dr. Mid-Nite doesn't change much at all. His costume could be made from fabric and leather quite easily. I might make the jerkin heavier, like actual armor of some sort (light armor, though).
GL also stays pretty much the same. To dress him up a little, I think it might be neat if he wears a tabbard with his badge on it. I might also change the cape. The heavily starched collar would be pretty annoying in real life, so one with a lower collar might be better.
Wildcat was also pretty easy. I imagine him as kind of a merchant marine type guy, so he's got one of those neat black sweaters with the leather patches on the shoulders, and some pants. Pants are very important.