The Marvel Cinematic Universe exists on the big and small screen, but there's hardly any crossover, meaning we'll never see a number of iconic pairings and team-ups. At first, it looked like MCU movies and TV shows would have a tight relationship, with references and crossovers aplenty. However, as Marvel Studios and Marvel TV gradually drifted (the movies report directly to Disney, television Marvel Entertainment), that promise hasn't been delivered on.
Agents of S.H.I.E.L.D., Agent Carter, all the Netflix shows and everything since have all existed in a sort of MCU purgatory, referencing big events like the Battle of New York but never making a dent in the movies: Daredevil sat out Captain America: Civil War, and while Avengers: Infinity War featured all the Marvel characters you could shake a stick at, there were still no TV characters to be found. This is only going to get worse going forward with the announcement that Marvel Studios is producing its own TV shows for the Disney streaming service, essentially relegating the Marvel TV efforts to a third tier of continuity importance.
Because of this inter-company divide, we're unlikely to see Marvel movies and TV ever properly cross over - and that means some iconic comic team-ups will never be seen on the screen, big or small.
Daredevil and Black Widow
One of Marvel's more famous romantic pairings was Matt Murdock and Natasha Romanoff. Their on-again, off-again relationship - born out of Karen Page leaving Matt Murdock, unable to deal with his costumed alter-ego - has been beset by intriguing twists and turns ever since coming together in the early 70s.
There was an engaging love triangle between Daredevil, Widow and Hawkeye that fans fondly remember. Widow and Hawkeye's bond predates Daredevil, as Romanoff was still loyal to the Soviet Union at the time and attempting to turn poor old Clint Barton for her own nefarious purposes. Eventually, her relationship to Barton helped change her mind to defect, and later she joined the Avengers. Over time her and Clint broke up, and Murdock didn't fare much better, with Natasha ending their relationship to focus on her duty to the Avengers.
The MCU hasn't really known how to handle Natasha's relationships, eventually pairing her with Bruce Banner in Avengers: Age of Ultron. However, that surely comes from Marvel's Netflix shows being kept separate with little crossover. It really would have been a treat to see Matt and Natasha engage in a relationship on the big (or the small) screen; even Daredevil star Charlie Cox was vocal about his desire for Scarlett Johansson to make an appearance on the Netflix show.
Widow's naturally duplicitous personality from the source material combined with Daredevil's preternatural abilities to as a human lie detector made for good storylines, and their combat styles would no doubt make for some of the top actions scenes in the franchise.
Daredevil and Spider-Man
Daredevil and Spider-Man have always had a thoroughly entertaining big brother/little brother relationship, with Murdock often getting annoyed by Peter Parker's never-ending babbling and less refined fighting style. The heroes, as much as they bicker whenever they team up, have a strong friendship, and they've even traded costumes a few times to cover for each other's secret identities - notably when Murdock was outed by the press as being Daredevil.
Charlie Cox has stated how much he would like to see a DD/Spidey team-up as well, although that's a no-brainer. The pair would look immensely cool bounding across NY rooftops together, and considering their shared nemesis in The Kingpin, or continued run-ins with The Punisher, there's a team-up just waiting to happen. Kevin Feige did say it could happen "at some point", although that's rather unlikely in any near future.