That would be a bit weird, it wouldn't really make sense. The character Colin Farrell was playing was the head of MACUSA Magical Law Enforcement who Grindelwald had killed, and Grindelwald was presumably transforming into him via the usage of polyjuice potion using hair or other genetic material he had taken from the corpse. Fantastic Beasts 3 is set many years later, there is no way he would have enough genetic material left to continue transforming into him with polyjuice potion, unless he had captured him instead instead of killing him, not to mention he would have no reason to transform into him anymore, since he had already revealed his identity and was leading his army as himself in the 2nd movie.
Better to replace Johnny with somebody who looks enough like Johnny's Grindelwald for it to not be an off-putting change.
Sadly Disney had already dropped Johnny like a year ago, and announced plans to reboot Pirates of the Caribbean instead of making the planned final movie with the return of Davey Jones, Will Turner, and Elizabeth Swann, that they had set up at the end of 5.
Ya I just thought Colin Farrell deserved a bigger role :P
And that's really unfortunate about the final movie of Pirates because i really wanted to know how that was going to go down. I know Disney is strict but couldn't they just let Depp end the series off properly with one final bang. T_T
Yeah, Disney did him wrong, and now WB did him wrong. A part of me hopes it blows up in their faces with people mass boycotting both the Pirates reboot and Fantastic Beasts 3, especially if Johnny wins his lawsuit against Amber next year, but there is also a part of me that hopes that Fantastic Beasts 3 succeeds, so that we can get the planned 4th and 5th movie. As for the Pirates reboot, I really hope it does flop, reboots are usually terrible anyway, and since this reboot is only happening because Disney screwed Johnny over, I want it to flop all the more.