Usually, cutting out a love triangle is a good thing. In this case, it might have actually been a good move to add it.
According to Stanislav Ianevski, who played Quidditch star and Durmstrang champion Viktor Krum, his character originally had a much more interesting reappearance in Harry Potter and the Deathly Hallows Part 1.
In the books, Krum is a guest at Fleur Delacour and Bill Weasley’s wedding. And, yes, his presence is enough to set off Ron’s jealousy. Ron pulls Hermione onto the dance floor before Krum gets a chance to ask. Director David Yates, however, had a different plan for the film. Said Ianevski on the EW’s Binge podcast:
He [Yates] invented a new story that wasn’t in the books, a love triangle between Viktor Krum, Hermione, and Ron, but that was cut out of the film because it obviously didn’t fit with all the horrible things that happen in the last book.
... We did a new dance scene, I sort of stole [Hermione] from Ron. She remembered our old times, Viktor was acting like a gentleman again, happy to see her, she was happy to see him. Ron was sitting on the side, was jealous, and then Viktor took her to the dance floor.
I like it. It gives Ron an actual reason to be jealous and gives his later reaction to Harry and Hermione more dimension. Plus, it seeds that jealousy into something that can grow into the totally out of control emotion we see later in the film.
It would have been especially useful in the film since, unlike in a book, there’s no text to remind people of Ron’s jealousy problem. It’d been years since Goblet of Fire came out, so the clearer they could make Ron and Hermione’s budding relationship and the problems it has, the better.
Ianevski joked that Krum could have “gotten into a fight with Ron, and Viktor would obviously win and then live happily on forever with Hermione.” Well, that’s just taking it a step too far out of canon. But the rest of it? Could have worked.