I'd say that this is one of the big reasons why the first movie didn't work. The scope was too large, it was like a child taking all their action figures out and smashing them together. Almost a dozen "important" characters and multiple locations that were not fleshed out so there was no sense of scale or continuity. At its core it wanted to be a war movie, but since so much was just thrown at the audience at once it was never really clear what the stakes were.
The Arthas story could be a lot more simple. Lets say you just want to do from the discovery of the plague up to when Arthas kills Mal'Ganis. You have 6 main characters (could drop Jaina from that part of the story but then it's just a sausage fest) and only a few distinct locations none of which require all that much backstory for the action set pieces that take place there. The plot follows a pretty simple formula (revenge corrupts) that revolves around a single character and all the others serve to propel that one character's arc. No need for convoluted side plots and romances.