There are a large number of level 50 MSQ quests and dungeons that need to be done before you can start on Heavensward.
It will be the same when you reach level 60 - a whole bunch of level 60 stuff to do before you can start on the Stormblood content.

It can be a good idea to get the expansion even before you are able to start the content, because without the Heavensward expansion you will not gain any exp past level 50. If you do have the HW expansion registered you will earn exp from all the level 50 content and likely be around level 52-54 by the time you enter HW - which can really help in the first areas.