Just to share my experience in leveling alt right now. It is totally possible to level an alt from level 100 to level 110 just using Legion invasion alone. I have already leveled a priest this way since 7.2 dropped. And I did not even do the starting artifact quest, which I intend to do after reaching level 110. This expansion has a lot of story which can only be experienced by having all 12 classes, therefore I am leveling all of them. Without the artifact quest to unlock other quests, can still fly over to the main land to grab FP and do the invasion world quests when they pop.

More good news on the way is that according to dev, heirlooms will be upgraded in patch 7.2.5, which will make leveling alt even faster than.

Wow!
Guess I have to get my flying first before getting an alt?

It will be more effective to level alt after you have flying. If you don't have flying, and your alt has not done the beginning artifact quest, you are stuck in Dalaran unless you are willing to glide down and run to a FP to establish a travel path from Dalaran. And getting to various invasion world quest means a lot of running on the ground while flying is usually a direct path.

Legion invasion happens every 18 hours or so, each with 6 hours duration. During the invasion, just fly the alt to the invasion area and participate. The easiest is solo boss fight, other players will come and everyone gets to tag and bring down the boss. The fight is scaled to each player level just like the pre-expansion invasion. Some world quests are more complicated and harder to do with a leveling alt unless there are plenty of players around helping out. So I currently just do the easier one. With several alts being leveled one after another, each invasion have plenty of things to do.