[bitwize]

So I know that everything sounds cooler in Japanese (which is why anime characters call out their ultimate attack names in English), but is there a reason we can't use the perfectly cromulent English word "vocation" with a bit of explanatory text differentiating it from "passion"?

[cle]

Because everyone has different emotions, preconceptions, etc. attached to the existing word. Introduce a new one, and you can define what it means without worrying about people misunderstanding you (unless they already know what Ikigai is, and I’d be willing to bet in that case that they’d disagree with the author’s over-simplification or mischaracterization of the word). There’s also an “exotic” attraction to concepts from other cultures.

[defterGoose]

Translation is inherently a leaky pursuit (and I'm no native Japanese speaker, so I can't unequivocally translate the word 'ikigai'), but I can say as a lover of language that the value of such a word to a foreigner is it's beautiful concision.

[karmakaze]

The graphic linked by kybishop includes 'vocation' as well as the other intersections and descriptions of the four unnamed areas of overlap.