Something worth mentioning if you were previously in a "rush to endgame" mindset and now intend to slow down and enjoy the journey: you may have rushed past "ex-endgame" content at the old level caps, but these are worth doing - and best done sooner rather than later to keep some things in the intended chronological order. (If you go back to play level 50/60 content at this point, it will still be set in that timeframe of post-ARR / post-HW and not where you currently are in the story - eg. the Binding Coil would be "the first time you meet Alisaie since the start of the game" even though you're travelling with her now in Stormblood.)

Major endgame quests at Lv50 are the Binding Coil (first quest is given by Urianger - pick up all his other quests too) and the Crystal Tower alliance raids (starting in Mor Dhona). CT is easy to get into via the duty finder, but for Coil you'll need to use party finder and do it unsynched. Likewise for the EX primals.

At Lv60 you have the eight-man Alexander raids (in Idyllshire) and Warring Triad trials (Ishgard near Fortemps Manor), and Sky Pirate alliance raids (Ishgard airship landing).

Once you get to the end of Stormblood, you'll have Omega, the Four Lords and Ivalice.

TL;DR: do any sidequests with a blue marker and plus sign, which indicates that they unlock something - either a dungeon or a game feature.