I really don't think this should be about big investments. That's what everybody seems to focus on (we're not buying enough! we're not buying the right players!), but that's never going to be enough. Liverpool have a few great players, but overall, their squad isn't awesome in every position; and some of those that look awesome now, would probably 'just' look good in other teams. (Just like many of Klopp's Dortmund superstars couldn't make it in other teams.) Rather, Klopp and Pep have found a reliable way to pretty much beat every single non-top team every single time. That's not down to players, that's down to tactics. Of course, you also need players that are at least pretty good, but United has a lot of those already. Undoing packed defenses and towing away buses won't suddenly happen if you just buy Sancho, or whatever potential superstar is out there. To really make that next step and get some 20-30 points more next season, Ole (and same for Lampard at Chelsea) will need to make a tactical leap.
The problem is that it's really a huge leap; except if Liverpool and City slow down a lot next year, every title challenger will pretty much need a 100% win record against the bottom 15 teams, and good to great results against the rest. I don't know if Ole has it in him to get there. I don't mean that negatively; I just really don't know. But if he doesn't, I don't think you can blame him. How many managers out there have ever accomplished this sort of relentlessness that we're seeing now from Liverpool and City? Still, if United want to challenge, I think that should be the focus, rather than just buying a couple of Sancho's (in different positions) and putting them all on the pitch together. (Although as Pep himself admitted, you certainly can't do it without good players!)