I just saw the promo picture and here is what I think:
If you notice in the picture you see two clock towers, one is dark while the other isn’t. What stood out to me is that they are two which might stand for a parallel universe or a different time in Mystic Falls and that’s where Bonnie and Damon will be stuck.
We see Stelena holding hands which might represent Stefan guiding and helping Elena get back on her feet. Stelena is also close to the tower that has “light” which might stand for Stefan guiding Elena and helping her come out from the darkness. Throughout the season Stefan and Elena might spend time together and hopefully we get some moments between them. It can also stand for Stelena to be looking for Bonnie and Damon.
We also see Bamon holding on to each other. To me, this represents that Damon and Bonnie will get closer throughout the season. Also, they are on the side where the dark tower is and it looks like Bonnie is grabbing on Damon while he is “protecting” her from the darkness. They will both be stuck in another universe not knowing what is there, who is there, nothing and they will need to protect and work with each other if they want to survive.
Last thing, Caroline is on the floor and she looks confused, lost, and abandoned. I feel like this season Caroline will be on her own and have to figure her problems out. She is closer to the side where the dark tower is and to me that might represent a new side of her, dark Caroline. Another option could be she being heartbroken by Stefan. If you notice, everyone is running away from the clock towers [Mystic Falls] and Caroline is the only one “holding” on to the ground. Perhaps, this season something will happen to the town and Caroline won’t be able to let go of it.
This is what I think will happen on The Vampire Diaries season six.