As people like to say: "There is no canon." This means that there is no "main storyline". Reason for this is because everyone adding on to one solid block of story would be insane, especially with how many authors we have now. It would be very restrictive and arguably difficult to add to. If one person contributes to it often, those that contribute less would be interfering with what is a majority of the baseline work.
However, with how things are now, one could say there are "infinite canons". This means that everyone is at liberty to have their own idea of how the Foundation works. The Canon Hub is the prime example of that, showcasing multiple storylines that very well could contradict another canon (or work well with one!).
TL;DR: There isn't a main storyline, but there are a bunch of different storylines that are just as worthy of belonging in the SCPverse.
