There's one character I was really hoping they'd include from Fantastic Beasts, and that was Dumbledore. He's one of the biggest characters in the Potter universe, and completely under represented in the Universal Wizarding World lands.
They could make a show which starts with the older / present day Dumbledore. He regales the 'students' (audience) with a story from long ago... Casts a spell, and *poof*, we flash back to the 20s with young Dumbledore played by Jude Law. We see Dumbledore in his prime, in a show full of action and of course magic. Wand battling with practical effects, screens, and stunt actors.
The attraction design could be a sit down theater like Bourne, or a walk through like Poseidon's Fury. Or heck, the puzzle theater design we've yet to see used.
The story itself wouldn't have to involve Grindelwald, as Dumbledore is sure to have many more stories beyond what's told in Fantastic Beasts.
Whether they do this concept (or something similar) at USF or EU wouldn't bother me either way. Since the audience is taken back in time through time travel, they could also be brought to anywhere else on earth. So even if the attraction is 'in London,' the time travel story could take place in America or Paris, for example.