I don't know if someone picked that up already, if so I apologize, but here are my two cents on Tonoyasu.
I was reading again the last chapter (940) and a wild but soon really plausible idea crossed my mind.
Watching Tonoyasu, I couldn't help but notice that he is physically just like the Shogun, Orochi.
Here, you can see for yourself :
Same frown, same wrinkles, same lips, same "smile structure".
But the thing that really leads me to think that they are related is their hair
Below, you can see that they share the exact same hairstyle
Oda also drops a hint through Kanjuro and Shinobu stating that he might be more "important" than his humble demeanor suggests, giving to this speculation a little more weight
Although this last thing I noticed is weak, here it is as a bonus.
There is also another physicall trait of Tonoyasu that might suggest that he is related : his "crown shaped eyebrows" that are reminiscent of Orochi's crown
If Tonoyasu is indeed a relative of Orochi, it could give insight into his motives : he wants to right the wrongs his relative done to the people of Wano by helping them, tending to them. This might also explains why he wants to participate into the alliances rebellion.
That's it. What do you think about it ?