People expecting Nagito to show up, how the hell would he have gotten off the island and orchestrated the events?
It's very simple: he's Nagito. More directly, he definitely set up the killing game... before 2, anyway. He doesn't need to be there following it. Essentially, all he needs to do is set up a row of dominoes and wait until someone sets them off. We know that this is in his skillset - he set off an explosion, and said triggering required:
1. That Ruruka would need a similar drug
2. That he would need the drug in the first place
3. That he would fall and fail to remember which drug was which
4. That both Seiko and Ruruka didn't realize Ruruka had the wrong bottle
5. That he would run into Seiko, who owns the same kind of bag
6. That he would stumble, spilling one of the tainted drinks
7. That the dog would seek out Ruruka, Ando, and Izayoi
8. That this would cause Ruruka and Ando to fall on the trigger
Everything worked out
perfectly to see Nagito's will enacted, and he avoided expulsion in the meantime. Even the punishment he received helped him, as he stumbled upon the firearm he had.
If Nagito isn't in Future, it feels like his arc just ends way too abruptly.