What do you think will happen to unofficial slender games now that the arrival is getting a remake/sequel?

It depends really. Although, it seems unlikely that Slender: The Arrival might get a sequel. So it's either a remake or a remaster since the title hasn't changed and the teaser shows Charlie guarding the stairs inside the burnt house at the end of the game.

Parsec Games/Blue Isle Studios do not own Slenderman, so it would not make sense for them to take down other projects.

No..? Why would they do that if there were thousands upon thousands of unofficial slender games released before and after the original slender the arrival released and none of them were taken down?