What do you think of the theory that Aegon will be presented with a choice of either marching west to join the fight against Euron, or taking advantage of the chaos caused by the combination of Euron’s invasion and Cersei’s incompetence to seize the Iron Throne?

I don’t think that’s right. Aegon’s taken Storm’s End, next stop is King’s Landing, that’s where all his plot and theme dynamics are pointing. 

GRRM already has his pieces in places to deal with Euron and they’re called Garlan Tyrell, Willas Tyrell, Samwell Tarly, and Leyton and Malora Hightower.