This weekend at GP Brussels a couple slight variants of Abzan made it to the elimination rounds. One splashed for Exert Influence, and the other adopted Heir of the Wilds as its two drop. Exert Influence is a great include that can be splashed very easily, but I've chosen to highlight the Heir of the Wilds build as it's very close to the build that I have come to over the last week of testing on Magic Online.
I can't speak to Bart's preparation, but in my testing Hangarback Walker was the one card in the deck that didn't allow you to easily commit to being aggressive or controlling. Most of the deck is great at switching gears, but Walkers are inefficient when it's beat-down o' clock. It was too slow in situations when I decidedly needed to be the aggressor and it basically only ever chump-blocked twice against the red decks. I started out with two copies of Heir of the Wilds in my build, and it didn't take long for the number to become four.
Anafenza enables Heir to attack for three as early as turn three, and unless it jumps in the way of a first-strike creature there's no way to stop it from trading when it blocks. When it comes to Abzan mirrors and matchups that tend to go long, it's an obvious card to board out, but the game one applications have been very positive in my testing. I will say that I'm suspect of Bart cutting a Gideon, as he's your best tool against Ojutai's Command, but the other changes are easily defensible in my experience.