As always, it depends on how completionist you want to be. In terms of time limited stuff:
Do enough mythic raids / dungeons to buy all the remix mount / armor set recolors you are interested in.
You can buy remix housing items if you're a completionist on getting parts, even if you don't care about building your Barbie house.
Also note that you can just buy the fel recolor class mounts instead of levelling every class.
If you didn't get the Violet Spellwing back in the day, do a Mythic (+ maybe Heroic) Antorus.
Do a mythic Trial of Valor raid without dying on each armor type for its Funerary Dead set.
Do mythic +20s to get the skips to five(?) dungeons, if you don't have them and still want them for some reason.
Do a mythic +30 for an armor recolor.
Do a mythic +40 for a title.
Do a mythic +50 for the "Putting the Finite in Infinite" Feat of Strength.
After you've purchased / done all of the above you're interested, then check the vendors for mounts / pets / armors / whatever still available on Live you don't want to bother farming up.