I'm gonna make a "call" on this.
Arnold will be in it.
The silence from both parties regarding "The Predator" makes me think Arnold will be in it, some-way, somehow.
Shane Black was in the movie with Arnold. Respects him, knows the magic that happened in the first one is STILL unbeatable to this day how many years later. If Black is not negotiating, and/or begging for Arnold to at least appear in this film then...
Connecting dots, I believe Black was waiting for Arnold to have a "clean" slate. Arnold is done with "Maggie" and "Terminator" and "478" is in post production. His other two titles are "announced" but have no actual timeline yet. So his schedule should be "clear" in terms of shooting this.
I'd be shocked...and wrong for that matter.
But my gut is telling me it's gonna happen.