My two cents. And I have only considered those matches which were close
RG 2012 F - If Rafa would have lost that, it would have been a deadly loss of confidence and aura, which might have significantly altered the GS race, in Novak's favour.
Wimby 2018 SF - A loss to Novak would not hurt him much, as he was in a comeback mode, but would have bolstered Rafa's credentials on grass.
USO 2019 F - A loss for Rafa from 2 sets up would have been a heartbreaking one for him and dented his confidence on Hard courts. Likewise, would have announced with a bang the arrival of new Young champion (Medvedev) on the tour.
USO 2020 F - Arguably the most heartbreaking loss for a young generation star! Zverev might have reached greater heights, if he could have closed out the final. A loss for Thiem would not have mattered much career wise, as subsequently, Dominic just withered away due to multiple injuries
AO 2017 - A win for Rafa, would have firmly placed him in an unassailable lead in GS race, as he would have fed on an awesome aura of a winner. It would in most probable case, would have been a triple GS titles for him, and also with all the confidence, placed him in pole position to reach another Wimby final against Federer.
Lastly, many might not agree with this, but I believe that Federer beating Novak in RG 2011, was actually a disadvantage to Rafa, unlike what most of the MTFians believe. It is a known fact, that Rafa plays at a different confidence level at Philippe Chatrier and could have actually won against Novak in the finals and broken the mental block against him in 2011 itself. It would have assisted him at playing at a higher confidence level at next 3 GS title matches