What to do if I have already completed all the story quests ?

farm

Sit and be sad, until the next story chapter.

Really though, farm embers, QP, and ascension materials from daily quests and free quests. You'll always need more.

If you didn't complete all the free quests at least once do it for the quartz.
If you still have interludes/rank up quests to do take advantage of the dead time and do them.
If you did all of the above, while waiting for new content curse aniplex for putting so many dead weeks together. Seriously, nothing worthwhile happened all January, and beside the Valentine event February wasn't all that better.

As stated: Farm. lol. Work on obtaining mats for must have skills like steroids or NP batteries. You will help yourself long term in terms of gameplay and in grabbing FPs from people using your support.

If you don't want to do that run doors, QP goes fast.

If you don't want to do that either start to stack up embers in your second archive.

Depending on setups you can do some of these farms without spending much time playing the game at all just burning natty AP.

Pretty much what everyone said. Go be a farmer :P

Sometimes I try to think up new teams, especially teams that use 1-3* then test them against Gawain in Camelot's free quests.

I rather like the down time between event grind fests. I can relax with just a few qp quests and catch up with real life. Back to back events and chapter releases lead to fgo fatigue.

Farm and level up your servants. Have them ready for the next event/quest.

What you didn't know this game other name? Farm/Grind Ordeal