Hear me out, and let me know if I'm not the first:

I'm currently re-reading Oathbringer, and they've just had the breakthrough with the Dawnchant. Thinking on languages and the way they change over time and how it relates to fantasy series, and translations being imperfect to start with...

Dawnchant... Dawn. Chant. Light. Song. Lightsong...

Every one of the Returned who gave away their Breath died while they were Invested. This would've bounced them to a Spiritual Realm staging ground like it did for Kelsier. Kaladin sees paintings a la Warbringer while in Shadesmar, clearly intended to be interpreted by people who are invested. My guess is that the Returned (or their direct descendants) were the original human Voidbringers that settled Roshar

Following this line of thought and remembering the River Song/Melody Pond reveal in the Demon's Run arc of Doctor Who, I'm going to make an intuitive leap that the spirit of Lightsong the Bold was the identity of Honor