At the moment, when you're leveling a character through Broken Isles, some Legion invasion world quests and class hall missions will award you with a bonus XP item. If you complete these while your bags are full the item will get mailed to you rather than used instantly, and will grant you the XP when you take the item from the mail. You can farm these and use them all at once when the expansion is released.
These items still exist on the beta and work past 110. The mail has a 30 day expiry so you'd want to start collecting them a month before BFA launch. I'm not sure exactly how many levels you'd get, though I think it's a static amount of XP so maybe someone else could figure that out. I'm not aware if you can still receive these while XP locked, so farming dozens might be possible if you lock a a character at 109.
As there are no realm first achievements this time around I don't mind sharing this. I know Blizzard could change this before launch but it has to be done in advance to be useful anyway.