Let's assume a hypothetical scenario here and while I know players are against this potion on principle alone (and I respect your opinion on the matter, it's perfectly fair to feel that way) this is more directed to the players concerned about bad players in DF. What if the potion jumped you straight to level 50 and the trip to ishgard. But before you could proceed with the story quests, you went through a tutorial that went over how to play the class (no class is really that complex at 50 in my opinion) and what your expected role is. WoW did something like this with the heroic dungeons where you had to prove a certain level of competency/skill in a trial designed for your role before you could enter.

Tanks had to keep a dps/healer NPC alive against increasingly difficult mechanics revolving around aggro control and positioning, healers had to keep an NPC alive while worrying about the same, and dps had to burn down a variety of targets while knowing which ones to prioritize. While this will never replace actual leveling experience, I do think it would give boosted players an idea of how to play their class before jumping into content. Sky and Sea could very easily be adapted to something in the vein of this.