I haven't found any other forums that discuss exactly this, so I made a new topic.
This forum proved that the Nightwatcher could be/probably is Cultivation.
http://www.17thshard.com/forum/topic/38165-cultivation-is-the-nightwatcher/?hl=%2Bnightwatcher+%2Bspren#entry310555
So lets work with that assumption. We see that Syl, an Honor spren, refers to the Stormfather as her father. The Stormfather is also shown to be a part of Honor that fled when honor was killed.
Quick note: Based on the book, elemental spren are from Cultivation, and emotional spren are from Honor. It looks like there might be an equal number of Cultivation spren (Shadow spren, possibly Light Spren) and Honor spren (Honor spren, Cryptics, High Spren) in the Knights Radiant in the end.
So, since Wyndle calls the Nightwatcher "mother," what if he is a ..... Cultivation Spren! Wyndle is made up of vines, which you cultivate. Also, one of the surges he provides is growth and regrowth, which seem very reminiscent of the idea of cultivation.
Tell me what you think, is Wyndle a Cultivation Spren?