OK, so, I've been wanting to write this for around a month and just got to it. Heres my theory about the mount chilliad stuff.
As many of you know, the mural is located in the cable car building on the top. The lighting bolts people believe they represent thunder storms, however, I believe the mean something else. If you look it up, lighting bolts (is attached) also mean "White Supremacy". You may say, that those don't look alike, I believe Rockstar altered it so they woulden't get called racist. Thi sconnects to this because, if you look at the mural, theres two spots to connect to the surface besides the top point. I believe, if you get Micheal and Trevor to the spots on the side, it might do something.
Now on to the jetpack and UFO. There is a line, connecting the UFO with the jetpack. So, if you go to the top of it, when the UFO is there, and bring a jetpack, it also might do something.
In a comment by ExSlax (is attached) he sates, "3 protagonists (two old friends and a newbie), 5 mission categories to clear. Thunderbolts to show heavy rain." I believe, if you finish the whole game, and get Franklin to the mural/top of the mountain, Trevor or Micheal the jetpack side and the UFO side, have it rain, this could solve the mystery.
I'd try this, however my GTA save currupted so I havent been able to redo it.