With two Fantastic Beasts films under his belt, Gellert Grindelwald will feature in the next franchise film more than ever before. However, if those behind the franchise want to push him into the same league of scariness as Voldemort, they have to pick up the slack, and they have to do it quick. As noted by Screen Rant, the next Fantastic Beasts film needs to give viewers an actual narrative reason to fear Grindelwald. Simply labeling him as the bad guy, and expecting everyone to just go with it, isn’t enough to get him to “He Who Must Not Be Named” status. As of right now, we are totally comfortable naming Grindelwald.
The Screen Rant article also notes that Voldemort was an effective antagonist because his reputation preceded him. His reign of terror during the First Wizarding War solidified his place as a legitimate threat, and thus his constant background presence throughout the Harry Potter movies — building to his eventual return — gave fans reason to be concerned. We would also add that Voldemort’s very real threat is present right from the beginning in the orphaned Harry’s pitiable state, with a reminder of the Dark Lord’s attempted infanticide stamped right there on our protagonist’s head.
As far as Grindelwald goes, he “is a schemer still trying to rise in power, so when Fantastic Beasts: The Crimes of Grindelwald begins with a prison escape, the audience doesn’t feel apprehension or worry.” Basically, why should we care about this guy?