Actually, I never considered Luffy to be Roger's heir
Shanks is an ordinary human like Roger!
Luffy is the one who carries 800 years of history.Becoming the king of pirates is the smallest goal
Aye thats true, becoming Joyboy and the Warrior of Liberation or w/e, is something even beyond being pirate king, like an antithesis/counter to the tyrant who rules the world, Imu ofc.
Out of the two, Shanks feels more like a potential pirate king, Luffy is too young and Shanks is diplomatic, measured, chill, he does whats necessary when its necessary and tries to keep the balance and such. Hes fair, noble, just and obviously extremely strong too ofc.