They do have a long-term plan story-wise, which has so far encompassed Prime and WFC, that relates to the First 13... Something that they are still only revealing a bit each year (mostly at BotCon). It seems to be leading up to something, or at least sets a foundation for future (and current) series to draw from. It's just that each series (so far) is drawing from that foundation, but heading in slightly different directions.
(something we may continue to see for years to come - stories that are building on this foundation framework, but end up having a partial linkage to each other)