Season 16 ?

Finn is lying in a hospital bed unconscious, so your scenarios won't work.
Only if you assume that the movie picks up where the season finale ended.

In my imagination the movie takes place some time after the events of the season finale, maybe one or two years afterwards. Finn doesn't neccessarily appear in the movie. It could be mentioned in passing that she has returned to Seattle, DB leaves for DC within the first few minutes and our original cast (minus Warrick) are the focus. We get one fantastic mind blowing case that has everyone working together to solve (but no mass shootings and no CSI's in peril).

In theory the Nick Scenarios that stokesmyfire mentioned are possible. Mind you, Nick leaving a director's position to become a Supervisor is a bad career move and would not reflect well on him.

I see the movie as a stand alone and not as season 16 Episode 1.
Finn is lying in a hospital bed unconscious, so your scenarios won't work.
Well, given that CSI pioneered the "forecsic miracle" test, how do you know she just doesn't wake up, walk out and claim she had a vision from Eric Delko? Riddle me that.
I would like the movie to take place a couple of months after the end of Season 15. Finn is out of the coma, and D.B. has been approached by the FBI. However, the crew is disturbed that there is a killer in town that reminds them of a case that occurred 15 years ago. During the movie, we see Grissom, Catherine, and Stokes return to assist the crew on the case, and we are treated to such past storylines that tie to the cash that could feature Dourdan, Vassey, Brass, and several other actors who have appeared on the show over the years. However, I want the one perfect ending where Grissom and Sara decide to reunite, and she delivers the news to him that she's pregnant. That to me would be the prefect ending because it could set the stage for a CSI: The Next Gerneration series in another 15-20 years.
Great idea! It's possible that CSI could back in next 15 years - look that some of shows returns after huge break.
It's a very nice idea indeed, however a pregnancy announcement in the finale would be identical to that other show that also ended recently... (I don't want to spoil which one I'm referring to, just in case)
The Mentalist
Not that it should matter, but that other show beat them big in that department...
i'm sad it's almost over for the franchise, but it was retrospectivly a big foreshadowing that danson took that role on fx.i do not doubt that it was planned long time ago, and to toy with us fans just installed nick's depature for the thrill of it.lame.

so for the movie it will be difficult to involve the whole team for espacially one case, so my concept would be an characteristic TWO case movie, that however involve warrick and showlight that chemistry that made csi multiple times the most watched series world wide.