I actually think Nahel Bonds with singers were always possible, they just didn't do them for some reason. Maybe something to do with why Venli's spren can only speak thru the rhythms.
Question:
If we somehow gave Nightblood allomantic powers, once he started burning could he keep draining even after runnig out of metal because of his other properties.
I have a fan theory that Highspren choose radiants that can reach the third oath but don't understand enough about what the oaths are trying to teach to reach the fourth oath.