I have seen a lot of speculation around the site about how the OT and FST will be unified eventually. I believe this as well, but I had some thoughts on the means of this happening.
Many people are saying that the FST characters experience flashes of the OT when they encounter someone they loved in the OT. I disagree that this is a general condition. I think that in some characters' cases, their meeting and/or falling in love with their counterpart is a distinct element of the OT that helps them bridge the gap between the two. Hurley and Libby, Charlie and Claire, Desmond and Penny, Sawyer and Kate. They're all bound by love in some form, but love itself is not the catalyst for bridging the OT and FST. Everyone has a different "constant" that appears in both timelines. Love just happens to be a common one.
The key is Locke (no pun intended). Why would Desmond try to run him over? Locke does not have a romantic counterpart tied to the island. OT Locke was in love with the island itself, I think.
I believe that Desmond is trying to nudge the events of the FST so that the conditions of the OT are recreated. Emily Hawking once tried to explain this to the O6, that in order to return to the island, they all had to go together, and that the original conditions had to be recreated as much as possible. Desmond is doing the same thing here, perhaps without even really knowing why. He is taking Eloise's role in trying to get everyone to return to the island, except now they need to return through time and parallel universes rather than just space.
OT Locke is dead. If FST Locke died or had a near death experience, that may be the single event that allowed him to bridge the two timelines. OT Locke was paralyzed and later was healed. If the paralyzed FST Locke is healed and able to walk, that may also bridge the timelines.
Jin and Sun love each other in both timelines, as a pre-existing condition, having nothing to do with their time on the island. This tells me that "love" is not always the constant. FST Sun didn't start to "bleed over" into the OT until after she was shot and was forced to tell Jin she was pregnant. Her pregnancy, obviously considered a kind of miracle in the FST, could be her "constant".
Just something to chew on.