This is DC. You can bet that some writer will make use of ''Wonder Woman in the Golden Age'' in a miniseries, one-shot, or hell, even in the main title at some point...even if Johns' doesn't do a lot with it in his book.
Wonder Woman is the one member of the Trinity who has historically retained her ties to WW2 and the Golden Age. Of all the Trinity, she's the one character whose book reverted to the Golden Age/E2 version during the 70's for a short period of time. She's the one Trinity character who frequently gets stories set in WW2 (Legends of Wonder Woman for instance - haven't read it yet but heard great stuff about it). No Superman show has given us a 1938 Superman (though I wish they would!) but Wonder Woman got a season set in WW2. And her big cinematic debut swapped WW2 for WW1 but the inspiration for it to be a wartime period-piece was very much the Golden Age.
I dunno why that is, but it's very much a part of the character's real-world history in the comics and other media. And I say it's a strength, or rather, it can be if done right.
There are a lot of questions about how WW's mythos works with this new timeline. I say, bring it on. Let's have WW2-era Wonder Woman nostalgia, inspired by Marston! Let's have an epic miniseries about Wonder Woman through the decades! Let's have some crazy story filled with timey-wimey and weirdo mystical stuff to potentially explain two Steve Trevors! This is comics...it's supposed to be FUN!
As far as Power Girl goes, to me it's pretty straightforward - a Symbioship shows up in 1976, as a temporal/multiversal anomaly and crash-lands. The JSA find a young woman in it who vaguely remembers her name is Kara but knows little else, and then discovers she has superpowers. They take in this amnesiac, give her a home, and she builds a new life for herself as Karen Starr and Power Girl, serving as one of the powerhouses of the team. Decades later, she meets the newly-debuted Superman, it triggers some repressed memories, she thinks she might be Superman's cousin from Krypton, but a test proves her wrong. Years after that, she discovers her true origins and remembers her life from a previous timeline and her Superman.
See, not so hard is it?