Yeah, this is fairly dependent on what it is that you enjoy. I cant set a proper goal for you to enhance your playtime without knowing you. So instead, lets try this.

Do you enjoy having the most options available to choose from? Advance the MSQ and do all unlock quests.
Do you like making things? Level a crafting class. Want this to be harder? Dont use Ixali or Moogle tribe quests, limit leve and collectable turnins.
Would you rather make all the things? Then level all 8 crafters.
Does the idea of going out and finding materials to work with appeal? Level mining and/or botany.
Want to make money? Some combination of gathering and crafting is the easiest.
Do you like pointlessly wasting time and occasionally being rewarded? Try fishing - either in game or IRL.
More into fun and games? Save up to buy a Fenrir mount(1 million MGP - for most people thats a lot of cards or chocobo racing). You can also get the Archons Throne and Korpokkur Kolossus if you want to stretch this goal(another 750k each).
Want a deeper understanding of combat? Level and use a class of each roll - healer tank and dps. Bonus points if you expand "DPS" to: melee DPS, ranged DPS and magical DPS(for a total of 5+ classes leveled).
Want a challenge in battle? Get to end game and move into the Extreme/Savage/Ultimate scene.
Want a challenge you can try now? Complete a 200 floor run in PotD.
Want a better ending to the ARR storyline then the one Praetorium offered? Run the Coils of Bahamut. Find groups to do these sync'd/min ilvl for 2 more challenges(challenge one is finding the groups, challenge two is completing the content).
Want a better understanding of the world? Do every sidequest everywhere, and pay attention to all the lore that comes from it. If you do, bring more then one class to spread the exp around.
Want to do a light breezy sidequest line that most people agree is fun? Hilde!
Want to leave a mark on the world thats as permanent as you want it to be? Buy a house(easier said then done) and decorate it in a truly unique way.
Are you ready for the TRUE endgame? Glamour, glamour, glamour.
Like collecting? Work on one/several of the in game collections - mounts, orchestrion rolls, minions, and Triad cards. Note: If you are an obsessive completionist i advise against this goal, as some of these are more or less unavailable, others are only available on the cash shop, and many of the "available" in-game ones require insane amounts of RNG. In the event you do go down this rabbit hole, Frizze will not be held liable for any loss of sanity.
Like grinding? Work on some relics and animas!
Like grinding in a slightly more social setting? Centurio Tiger ALL the way(achievement reward for doing hunts - 5000 combined A and S ranks).
Want to improve the player base? Write a guide/make a video for some piece of content that people are having a hard time with that spells out how to win - this can either be general or from a specific viewpoint. Or find an area or two you feel comfortable giving advice on and help other players either in game or on the boards. Or recruit new people into playing the game who would be good at it. Or find a way to make the people who dont care magically suck less. Or maybe something else. This one more of a conceptual goal.
Not seeing any ideas that appeal to you? Try to use something on this list to inspire a personal goal you can work toward.
Want a longer list then this? Sorry, but im running short on ideas right this minute. But this seems like a good start. And in the unlikely event youve already done all these things, then teach me time management. Because ive been playing for a few years and i still dont have all of them done.