I try and think about what things from the current expansion that are sort of easy to come by now won't be in the future. An obvious example is the Steelbound Harness
mount from Blacksmithing. Costs 50 Blood of Sargeras
and a bunch of other mats to make. I imagine people aren't going to be out farming Blood of Sargeras
in the next expansion. Most people aren't going to have lots of it on a single character let alone one that is a blacksmith AND has the recipe from Nighthold. So that mount is going to inevitably going to rise in value.
Another one is the battle pet Hungering Claw
from the Kosumoth the Hungering daily. Lasts two weeks I think, decently long set up to even get the world quest. No guarantee it'll even spawn the next week. Could be the mount, could be gold, could be the pet, etc. It's really cheap now, that too will go up in price.
I personally sell alot of mog so I'm going to be stocking up on felslate and other trading goods. Legion 850 stuff uses alot of materials compared to previous expansions.
Those are two examples off the top of my head, I'm sure there's more.