I’m playing Season 9 Adventure League. I was fascinated by building a Assimar Barbarian character that could fly at level 5 according to the new rules in the Players Guide 9.1 Inglorous Redemption a campaign supporting Baldur's Gate Descent into Avernus.

“Plane-Touched Wings If you wish to create an aasimar or tiefling, that character gains the benefit associated with their race, below. Neither benefit is contingent upon selecting the referenced resource as their +1. Aasimar. You can choose aasimar (Volo’s Guide to Monsters) as your character’s race. Additionally, at 5th level, you can permanently replace the Light Bearer trait and racial trait they gain at 3rd level to sprout feathered wings—gaining a fly speed of 30 ft while not wearing heavy armor.”

Any thoughts on how to improve the following build up to level 12?

Race: Fallen Assimar - +1 to Strength

Class: Barbarian – Eagle totem spirt would give Dash on bonus action while raging

Weapon: Glaive for extra reach

Movement: 50ft because of the Fast Movement and Mobile feat additions

Feats: At 4, Mobility for flybys without provoking opportunity attacks and extra movement. At 8, GWM for the extra damage using reckless attack to better odds. ASI at 12 for improved Strength or would it be better to get Charger so the +5 charge negates the GWM minus to hit

The tactic would be to rage, move into place flying 10 ft above enemies and recklessly attacking with GWM using a Glaive to keep out of their reach. Next round begin flybys by Dashing with a movement of 100 ft. flying speed .

With a reach weapon you won't have to worry about opportunity attacks much, so the Mobile feat is not that necessary. I know the increased speed is attractive, but really anything past 35 ft/turn is unnecessary in my experience. So get GWM at our just spend those ASI's on stats. 

Flying barbarians are fun as grapplers too. You have advantage on Athletics checks to grapple while raging. Bonus action dash let's you counteract the half movement speed penalty when dragging someone grappled. Your damage from dropping someone is generally going to be lower than front hitting them unless you have a cliff or lava or the like, but it's just a fun tactic. A nice DM may allow you to drop one enemy on another and damage then both.

That's certainly a good point. But if I lose initiative and the enemy moves within 5ft I'm locked down. I would have to rely  on the Eagle totems disadvantage on opportunity attacks to get away. Mobile would allow me true flybys. If there were warriors guarding a wizard I could fly past them, attack the wizard and then dash fly away. However,  the problem there is prolonging the ASI advancements as you pointed out. It's a puzzler on what is best.

Quote from eedgejr >>

That's certainly a good point. But if I lose initiative and the enemy moves within 5ft I'm locked down. I would have to rely  on the Eagle totems disadvantage on opportunity attacks to get away. Mobile would allow me true flybys. If there were warriors guarding a wizard I could fly past them, attack the wizard and then dash fly away. However,  the problem there is prolonging the ASI advancements as you pointed out. It's a puzzler on what is best.

Mobile only prevents opportunity attacks from creatures that you melee attack, so you're right, but I hope you have a lot of melee attacks to throw out. But also bodyguards within 5 feet of your target can make things difficult .. except that you're flying and have a reach weapon. Fly 10ft over their heads and stab at them with impunity. Unless they also have reach weapons, but honestly, you're a barbarian. You can take a few hits, you'll be fine.