For Stephen to be successful as king and have his line be Kings in place of the Plantagenets he needs Empress Matilda to lose all influence in Britain and for Henry Curtmantle to die before he can become a major threat.
He also need his eldest son Eustace to die as OTL otherwise he will risk another William Rufus coming to power however in OTL it was the death of Eustace that convinced Stephen to make peace with his opponenets so that might not be a good idea.
So while his second son William is a better prospect for a good successful king Eustace may well end up taking the throne after Stephen and making the Blois line very unpopular