Just few bits and pieces of information:
- Cross-faction grouping is coming (except warmode).
- Cross-realm mythic raiding will open in future, just month after raid release.
- LFR wings open each week, instead of biweekly.
- Even more character customization (inspired by "amazing fans and creators all over the internet").
- All missing heritage armor sets are coming with prepatch.
- Heritage weapons coming later on with expansion.
- More timewalking love coming too!
- Plenty of QoL features added (including skips/teleports in older raids, improvements in legacy loot options etc.).
- Dozens, if not hundreds of cosmetic glyphs are added.
- Some of the removed questlines are added back to the game (like legendary questlines for cloak in MoP and ring in WoD).
- Big, new reward system added to one of the main features of the expansion. Similar to IE loot, includes plenty of unobtainable items, armor and weapons used only by npcs, items that are only in the files or even some of the highly desired, removed items from the past.
- Big shift towards customization, player expression, evergreen features and systems.
It's not a lot, sadly didn't get information about features, themes or systems of new expansion (besides the devs playing with ideas of rebuilding/modyfing talent trees).