I think Stormblood will require you to have completed the Heavensward storyline. However, don't worry, it's well worth playing. The story is really good
What might be worth checking out is the idea from dannykc83. If there's an option to buy HW+SB at the same time, that might save you some money.

And I just saw you're on my server, if you pick up HW and need help with anything, feel free to send me a /tell.