Could someone post a screenshot of the fire rasengan? I still have yet to see it.
I think we need to know the technical aspects of each element first though, like how Kakashi said wind is the most powerful mid range and cuts up everything to nothing. What if rock just crushes everything into pancakes? it probably wouldn't make a good Rasengan to be honest although I would really like to see what it would look like lol. And Kakashi couldn't do a lightning element rasengan cause he doesn't have the capacity to put the two of them together I think, he doesn't have the time or the chakra level or the understanding of mixing them like Naruto was able to gain.