Dynamic flying available everywhere would be nice, though might be unlikely.
Time walking revamp is definitely needed, but might also be too early. Maybe BfA time walking and MoP time walking raid.
Most likely I would guess it just a chunky patch with lots of small things done well, rather than one big central feature. Good transmog, interesting areas, loads of quests, class revamps, etc.
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I imagine it will be much bigger than 10.1. Probably some new materials like what we have gotten in large patches since Legion.