Probably wouldn't need them for Lyn when I've still seen alot of support for Lyn coming from Fire Emblem fans still.
Nintendo wouldn't need D.Va to fulfil that niche when they already got their own girl in a mech. A Warrior Princess with a Centaur Mech to be more precise.
That aside, while D.Va would be fitting for a crossover fighter about video games, if Overwatch were to get a character, I can't see it being anybody but Tracer. Sure, Spring Man shares a poster boy role with Tracer's poster girl role and he didn't get in, but one, Min-Min was more the producer's choice and I wouldn't be surprised if they would've added Spring-Man regardless of him being an Assist Trophy. And two, compared to Spring-Man who really only has the game's Arcade mode and 1 comic, Tracer gets alot more attention throughout Overwatch's story and background, being one of the more prominent characters used in the Cinematics/Short Movies, Comics and even a dedicated set of books.
While Overwatch tries to do a "No specific main character" mentality with it's message, "The World could always use more Heroes." and for the most part, they do a decent job, Tracer definitely gets more attention in media compared to the other Heroes and it would be no surprise if she was the first one chosen regardless of her poster girl stamp, unsurprisingly for the character who began the game's catchphrase.
But nowadays, it's hard to imagine Overwatch getting in, especially with it's recent reception nowadays. Poor Overwatch fans. And some for the dev team too considering Bobby Kotick was really pushing on them hard to do stuff that they would scrap later, thus wasting development time and preventing them from doing the more important stuff.