Here's the way I'd play it:
Make Sharon an old-fashioned good girl. She likes most of the stuff Steve does --- the 40s era music, movies, cars, fashions, etc. She volunteers to join up with Cap's crew out of a Coulson-like hero worship; but in Sharon's case, it's more personal, because she's grown up with Peggy telling her stories about the man, and she feels she already knows him.
Tasha, by contrast, is the 21st century girl. She's all modern music and styles and attitudes. Tasha and Steve often clash, usually humorously, over the worlds they grew up in.
Late in the movie, during "downtime," Sharon reveals her true feelings for Cap, and he, of course, realizes he's falling in love with her, too. Obligatory kiss, of course; then Steve needs some time alone to think things through. He's alone in his apartment when there's a knock at the door. Natasha looks sexier than usual, but she's here to talk shop and clear up some things about the mission. Things start getting hot and heavy, and she makes a pass at Steve...
And now Steve has to make a choice.
Of course, it's pretty clear who he'll ultimately choose; and Widow isn't going to be entirely heartbroken, because she knows that they come from two different worlds, literally; and that Sharon is right for Steve. But the movie can end with her making eyes at Bucky....
Yeah, it's all that mushy stuff that action movie fans hate; but it's the classic love triangle that creates the drama and romance that almost every movie really needs. I think it would work great.
.
.
.
