The biggest reason I don't see the need for a leveling potion is that this game doesn't have NEARLY a robust enough endgame scene to support the mentality of, "the true game starts at endgame". It works for WoW (kind of) because a.) factions actually significantly impact gameplay, b.) they place a heavy emphasis on endgame raiding and difficult dungeoneering to keep your character in the best gear, and c.) you actually have to level a brand new character to play another class, all of which are race-locked to boot.

XIV doesn't do any of those things, really. The best thing they could do is remove the MSQ gating for every little thing, and make things level-gated instead, so that once players get to the level where the "fun" starts they can jump right in.