Or even worse, might be his life span
Gear 5 makes his heart beat even faster, making him consume more oxygen
If Oda is going on a route that Luffy is too damn overpowered, i think the consequence is his death by the end of the series (yeah thats One Piece so probably wont happen)
Depending on how Oda handles this, i think its inevitable that Luffy might have to face an existencial crisis, something related to fate/destiny, that pushes him to the edges of his character