I don't think it will be about the creation of the traveller, but about trying to accelerate repairs on the traveller. I think the plot will center around the vex trying to infiltrate the traveller. The vanguard will be suspicious of any vex interaction with the traveller and of course will send us to kill them. Or it could be centered on the vex trying to create ANOTHER traveller out of the planet Mercury. If the theory that the vex are caretakers of the traveller is true, then the vex could be trying to make a new traveller since they don't have access to the one hovering over the last city.