There's a few things in both the casting and this trailer that leads me to think either the ending of T-2 is going to be changed via time travel, or we're going to see John Connor in a few sequences directly after the ending of T-2, and a few years down the line before he gets taken out by another T-800.
They have hired some kid to play John Connor at the age he was in T-2 and he looks very similar, so from that it looks like they're going to at least show some events directly after T-2. I reckon "Arniebot" is the one that takes John Connor out, he has battle damage so it would make some sense as in every other Terminator they seem to arrive in perfect condition (and not old). Legion succeeds\replaces Skynet and due to this it erases his programming or puts him in dormant mode as Skynet is out of the picture, he's reactivated and reprogrammed years later by Short haired Blondie. Sarah's reaction to Arniebot and telling him shes going to kill him sort of suggest this is the Terminator that finally took John Connor out.