Regardless, whether or not Sinister appears in the Logan post-credits scene, that still begs the question of which movie he’ll get to be the main antagonist.
New Mutants seems like the best bet since it’s the next X-Men team superhero movie that’s coming down the pipeline, but other options could include another main X-Men movie or the long-struggling Gambit.
As it stands now, the only thing we know about Mister Sinister in this universe is that his corporation secured a vial of Wolverine’s blood from Weapon X in the early 1980s, but considering the experiments that Transigen is running on young mutants in Logan, it wouldn’t be surprising if Nathaniel Essex was somehow connected to the organization.