I think a Black Widow movie can go big or small. Either way works.
The small route is underrated, with recent movies like The Fault in Our Stars and Lucy showing that you can make money with low or modestly budgeted films. Those movies were far more efficient than a lot of the big name, big budget disappointments we've had recently.
The MCU has succeeded by going small, in scope if not in actual budget. I think it's a mistake to think that everything needs to build into a grand overarching plan.Despite the dozens of Iron Man suits in IM3, that was basically a "small" movie about Tony Stark running around and fighting a bunch of human criminals. It didn't try to top The Avengers and did very well for itself.
On the other hand, I can see a "big" movie giving Widow a boost as well. Imagine if Nick Fury confronts Natasha in the post-credits scene of Age of Ultron and tells her that he needs her for a secret mission. Insert Natasha into the post credits scene of any other "Big Three" sequel if AOU is already taken up.
Right away, that sends the message that the Widow is a big deal, both as a character and as a part of the ongoing MCU story. It stirs up hype, and people will get the message that her movie is no standalone that they can just ignore.
The trailers can show Samuel L. Jackson and Jeremy Renner, and name drop the Avengers to play up the link. The hype for the movie can promise to settle the Widow/Hawkeye relationship once and for all, and explain her past and what happened between them in Budapest.
Then in the actual movie, we'd get big gunfights and car chases like TWS and the Daniel Craig Bond movies. Widow would be front and center, uncovering some scheme involving super soldiers, alien weapons on Earth, or another mysterious Tesseract-like MacGuffin (which will turn out to be an Infinity Stone). Natasha, or other characters, can remark that she's outgunned, fighting above her weight class. And it certainly looks that way in a big, explosive, visual effects filled climax. But she ends up kicking ass due to her grit or ingenuity, and looks better than ever for it.
Then the post credits scene could explain that Baron Strucker, Thanos, or both had been plotting everything behind the scenes, paving the way for another alien invasion of Earth. Sort of like how the 1990s Superman cartoon showed Darkseid prepping Earth by first arming human gangsters with advanced tech. This flows right into the next Big Three solo or Avengers movie.
That's just something off the top of my head. I really think the MCU is a great resource that allows for limitless variety, which can help a character like Widow reach high above the likes of Bond or Bourne.