Steve Rogers evolving to join Hydra? Potentially interesting. Particularly with the Red Skull turning into a Donald Trump caricature; you put in enough nuance to the situation, showing all sides to the issue, including the problems (Cologne!), and seeing Steve Rogers decide that perhaps helping Hydra might be for the best. If you toss in things like how much the world is changing, how much grey there is now in the world, etc., and show that Captain America isn't quite as necessary as he used to be, it could make for a very interesting story in the hands of a skilled AND considerate writer. There would still be a LOT of backlash, there would be charges that the writers were closet GOP/Trump supporters trying to make Trump/GOP more sympathetic, etc., there would be the cry that "Steve Rogers would NEVER do that!" when Steve Rogers said "I agree." to Red Skull saying "Many of these refugees aren't refugees at all - they are merely economic migrants!" or "These crimes in Cologne were perpetrated by migrants - with perhaps a few native Muslims participating as well." Even I might toss out a few "Oh, you." and "Really? Uh huh." gif's out on Twitter and message boards in disdain.
But to me, that would make more sense and be a more honourable way to treat this with Steve Rogers/Captain America than to say he has ALWAYS been part of Hydra. Instead of saying "The last 70+ years has been a LIE!" and making a mockery of the character, it could be a natural progression of an "old white man". IRL, you had millions of straight white males in the USA in the 1940's who weren't outright racist or sexist, and who would have risen up to support minority and expanded womens rights if someone had come along and actually pointed it out to them. They fought side by side with African American's in WWII and they depended on women making many of the things they used (including some weapons and vehicles) in their time in the military. But they simply didn't rise up to support people who were both racist and sexist because there were more straight white men who were sexist and racist, and those people had more incentive to keep things static.
So you have a Steve Rogers coming out from that era, being one of the people who WOULD support expanded women's rights and minority rights, but being a military man to the core (at the time) and less of a "Symbol of America" that he would become, he doesn't become that rallying point. He follows the orders of his superiors and doesn't get involved in politics. He supports MLK later on (though does harbour some beliefs that Malcolm X might be a better person for the civil rights movement to follow) and the expansion of women's rights. Later he supports the gay rights movement (but you can go back and show that he's uneasy with it because while 1940's white males might have had lots of people who would support women and racial minorities, homosexuals were still very much a taboo). But as things progress, the 1940's Straight White Male in Steve Rogers starts to yearn for a simpler time, when he didn't have to worry that saying "Coloured" was an insult (even though many refer to themselves as "People of Colour" - what exactly is the difference between "Coloured" and "People of Colour" anyway?), where he could compliment a woman on her looks and not be called a misogynist (I don't hate women - I love women. All women. What's wrong with saying that she looks beautiful in that uniform, anyway?), where you just didn't see two women kissing in public (women? They can't be more than 18, if that! They're still little girls!).
Being Steve Rogers, he isn't afraid to ask why. And he gets answers - but the answers don't satisfy the 1940's Straight White (Christian) Male in Steve Rogers that is making itself more known, in many of the same ways that you see some older people reverting back to a simpler time for them. (They work perfectly well for people who were born and raised during the past 30 years. They're the arguments that are used today and many people accept as valid. These aren't meant to be caricatures and straw-men which shouldn't convince Steve Rogers - the point of this is that he's started to reject "modern" thinking and reverting to a "simpler" time in his head.) And eventually this leads to Steve Rogers, incognito, going to a Red Skull rally and finding himself nodding at what the Skull is saying. And the more he see's and hears of what's going on, the more the Skull's appeal to that 1940's SW(C)M mindset starts to work. Eventually, under the guise of infiltrating to find out what nefarious evil the Skull is up to, Steve adopts a pseudonym and joins a minor Hydra group. Then from the inside, he finds that it's mostly SW(C)M's who have given into the racist, sexist and homophobic views of the 1940's SW(C)M, and it appeals to that part of Steve Rogers. You toss in a few situations - like another Cologne - where Steve Rogers witnesses police doing nothing (in his view, anyway) and when he DOES do something - like stopping a group of apparent migrants from molesting a group of women - the headlines the next day are "IS CAPTAIN AMERICA AN ISLAMOPHOBE?".
And he snaps.
Hail Hydra.
Now, because we don't want to make it too sympathetic to the Red Skull, we do find out that the Red Skull has been helping criminals from the Middle East, ISIS and Al-Queda, or their Marvel equivalents, come into Europe and cause trouble. He's got his fingers in a few police forces, not enough to be noticed but enough that he can influence them to not interfere in some "sensitive situations". And he's manipulated well meaning morons in the West to view any sort of criticism of Muslim migrant activities as vile, hateful speech and rhetoric, so people are scared of mentioning anything that is going on, which makes it easier for his criminal allies to operate. We find out that the "journalist" who wrote that sensational "Is Captain America an Islamophobe?" article was a Red Skull ally, and the article was written intentionally to hurt Steve Rogers. We find out that at least half of the talking heads on TV, Radio and Online who blather on incessantly about this situation are also Red Skull allies, and many are Conservatives using this article as a way to blast America for having gone too much toward the Loony Left. "When even Captain America - the Greatest of all Americans - can be slurred with "Islamophobe" for rescuing a woman from being molested - let's stop using that weasel word, she was going to be raped. Yes, RAPED - by an Islamist "Mi-Grant" - the PC way of saying "Terrorist" these days - when even Captain America isn't above reproach, what hope is there for the regular Joe American?"
And we find out that the end-game for this is for Red Skull to be outright elected to the highest office in the EU, along with hundreds of sympathizers in the other EU countries who aren't apparently connected, but once they are all elected they can turn the EU into a single nation with Red Skull at the helm. And what could be done about it - it was all democratically elected and sanctioned actions, so anyone attacking Red Skull over this would be attacking democracy itself?
And it works. Hail Red Skull, president of a united Europe. He has finally conquered Europe, only this time through Democracy instead of the Military.
At which point Steve Rogers would realize what it is he had done, what he had helped, and what he had supported. That the 1940's Steve Rogers mentality had allowed him to be so utterly manipulated by Red Skull that Rogers helped give Europe to the Skull... and there's nothing he can do about it. Steve Rogers is not equipped to help Europe. Captain America is not equipped to help Europe. We also find out that the Red Skull did NOT know that Steve Rogers had anonymously joined Hydra - that his attacks on Captain America via the Press were not to manipulate him into joining Hydra, but to delegitimize any actions he took against Red Skull in this. So when he finds out that Steve Rogers actually JOINED Hydra and HELPED him win? He's too dumbfounded to gloat. He actually thanks Steve Rogers in a sincere manner, no gloating, no speeches, no comic book villainy because even he's too surprised by this turn of events to know how to react.
And where does Steve Rogers go from here? He's just helped his greatest enemy achieve what that enemy has coveted for more than 70 years. He did it willingly and anonymously. He should have known it was wrong, but he was so blinded by something - and it's up to the reader just what that something was - that he just couldn't see that he was turning into a villain.
At the end, does he even see that he's a villain? Or has he succumbed to the idea that "Captain America is always right, so whatever Cap does is the right thing to do." that many Marvel hero's believe?