Levelling potion works in WoW but I honestly don't see it being a right fit for FFXIV, Blizzard is experienced with creating "tutorial" based content such as their honour grounds and they slowly introduce you to your class after you use the levelling boost. You're not dumped in with every ability, but a voiced instructor gives you an idea of the basics of doing somewhat decent at your roll and you do this for every character you boost. It works because at their core, WoW's classes are very simple with a complete ability set only taking 1-2 hot bars. WoW also has the excuse of older content being very dated and playing very differently to current content, FFXIV does not as it hasn't changed much since the beginning.

FFXIV in comparison is far more complex when it comes to jobs, every class has a ton of abilities that they won't be able to ease you into without a prolonged 2-3+ hour tutorial. Might seem like a poor argument until you look at what audiences the two MMOs attract, most people going into WoW are either a returning player or has at least played more than a few MMORPGs, whereas FFXIV attracts plenty of FF fans who've never touched another MMORPG. Can you imagine someone who's never touched an MMORPG getting pushed into a level 60 role with no idea of the basics? I don't see it working. There's also the issue with the story, WoW has never been as heavily story focused as FFXIV but I won't get into that.

I don't really know why I'm bothering to voice my opinion, I figured they were going to implement this and here we are, it's being "considered" and will more likely than not happen because of the money they'll make from it. They saw how profitable it was in WoW and that's it, they'll spin the new player excuse but as soon as they sell these on the cash shop rather than giving one boost for free with an expansion purchase that all goes out the window.