Eight hours in and I've already had two questions!
1 - Will we ever have a time travel mission?
Probably! Personally I find time travel missions have the potential to get far too complicated to be fun for everyone, and the subplots I've seen involving time travel have often been somewhat divisive among crews. That said, we've had several time travel missions before and when they've worked they've usually worked really well. If there is sufficient demand/a plot-driven reason for more time travel on the site, I'll certainly consider it in future.
2 - How far in advance does the creative team plan the story arcs?
That actually depends:
Some of the site's older story arcs were planned months in advance, while others might have been planned years in advance as culminations of several converging arcs. Currently we have no major story arcs past our site being set beyond the boundaries of the Federation and pushing deeper into the unknown areas of the Alpha Quadrant. That was an idea I had on the train home from work and discussed by the administration over the period of about a week; so that one was thought up, approved, announced, and implemented within about a month.
Individual missions on the other hand, are not necessarily planned in advance at all. People will submit ideas through the form provided, and if I like it, I'll commission them to write an outline for me. This could be weeks in advance of it being assigned to a ship, though it's not uncommon for a mission outline to lay unused for months or years before a CO accepts it. If there are special missions required for a story arc, they take a little longer to plan and put together, but I usually write those myself based upon the actions of the crew(s) in previous missions.