I was thinking - some of the big flagship supers look kind of funny outside of the setting of WW2.
Aquaman - invented in Nov of 1941 - spent WW2 fighting Nazi U-boats and Axis supervillains. The character makes a lot of sense in this setting. There's something meaningful to do in the ocean during that time after all. Since WW2 - Aquaman has been uneven. There have been some interesting stories and an interesting interpretation here or there. But for the most part - he's the member of the JLA that kind of sucks. I don't think anyone even knows exactly where Aquaman even is in the current DCU.
Wonder Woman - She was also created in 1941 and no one has really known what to do with her since VE day. She's wearing the American flag as a bathing suit. What can you have her do other than punch Nazis? She's about as dated as Rosie the Riveter. There's no real stories about Wonder Woman outside of her origin - which has her fighting Nazis. When they gave Joss Whedon a chance to do a Wonder Woman movie - he ran into this very problem.
"Besides [Wonder Woman's] great origin story, there's nothing from the comics that felt right 100 percent, no iconic canon story that must be told. Batman has it made — he's got the greatest rogues gallery ever, he's got Gotham City. The Bat writes himself. With Wonder Woman, you're writing from whole cloth, but trying to make it feel like you didn't. To make it feel like it's existed for 60 years, even though you're making it up as you go along. But who she, and what the movie, is about, thematically, has never been a problem for me. But the steps along the way, it could be so easy for them to feel wrong. I won't settle. She wouldn't let me settle." (Quote from Nov 2006 interview with Whedon explaining the delay in scripting a Wonder Woman feature)
(BTW - here's the link to the trailer for the soon to be released Wonder Woman animated film from Warner Bros Animation - - looks pretty decent from what I can see.)
Depending on which rumors you read - Whedon's (and other) scripts focused on Wonder Woman's origin - back in WW2. Which seems sensible - she's almost so completely of that time - she doesn't seem to fit cleanly anywhere else.
Captain America - The same issue - perhaps moreso. Can you think of one non-Avengers Cap story off the top of your head (besides his death)?
Cap - meh. Got nothing. I recall a few times when they tried to make someone else Cap in the 80s and there was a slug-fest for who got the shield. Maybe it's because he's got a lame Rogue's Gallery, but it's hard to get around the fact that in the modern age- he's an ensemble player.
The last time he was really a center stage hero was when he was punching Hitler in the jaw every other issue.
Having said that - Brubaker is doing a heck of a job on Captain America these days. He's taken the "Cap as a quasi Secret Agent" thing (which they've been trying to make work with Cap and with Wonder Woman since the 70s or so) and made it shine. Mainly through well-done action set pieces, lots of WW2 flashbacks and well, killing Steve Rogers. (Also - he's got a large and interesting supporting case in his book - Red Falcon, Black Widow, Sharon Carter etc - - which brings us back to the "Cap is an ensemble player" argument)
Maybe comics (for some segment of the characters - and perhaps the audience) is sort of permanently tied to the 1930s/1940s. That's when some of the flagship characters simply fit the best.