I agree that it would be an interesting fight and the 'revenge' narrative is a marketing department's dream...
Having said that, I will respectfully say that Robbie is well below what Nate is considering. Regardless of that, I also don't think the Diaz bros will 'step on each other's toes' by facing common opponents because it could get weird if 'little brother has to step up to avenge big brother's loss' (or vice versa, if Nick were to face someone who beat Nate). I just don't think they are interested in that kind of stuff...but money talks, so who knows.
Given that Nate doesn't seem inclined to fight before the end of the year, he probably waits to see how Dustin-Charles plays out, what Conor's return date looks like, etc. before making his next move. I could see him sitting until Conor is back or maybe pursuing Dustin, depending upon whether he wins the title or not.