So the Live Letter has revealed the next trial to be Sephiroth, one of the Warring Triads

As mentioned on reddit, FF7 Sephiroth was probably named after the Kaballah Tree of Life Sephiroth, so he's not exactly the originator of said name. In fact FF6's Sephiroth was probably named after the Kaballah Sephiroth too.

Now here's where it gets interesting..

http://imgur.com/gv0WfY8

The Warring Triad fiend was セフイロスin the concept art which is also how you spell FF7's Sephiroth's name in kana

whereas in the live letter he is セフイロト which is how you spell the Kaballah Sephiroth in kana

FFXIV's Sephiroth might be a combination of all those things!