If what you just finished was the fight at the Royal Menagerie, then continue the story until you get all the way to the current end point. That way you have the option to jump straight into the new expansion without waiting as soon as it goes live for you. If what you finished was the last bit of dialogue where a mysterious stranger tells you about the danger we are soon to face in Shadowbringers... well, if thats where you are then you have a decision to make. And that decision is what matters most to you. We have roughly a month. Thats enough time to do a lot of content, but maybe not all of it unless you play a lot. So do you emphasize trying out optional dungeons? Do you do one or more of the raid lines to see the story(Alexander, Omega, Shadows of Mhach, Return to Ivalice)? For that matter, had you completed the ARR raids before you left(Coils had a really nice story, Crystal Tower might be involved in the lore for the new expansion)? Are you more the type to work on a few beast tribes? Are you current with the Hildy quests(and do you care)?

Basically, what would be most fun and/or what do you want to accomplish most?