For the record, Sand might just be a form of Magnet Release. In addition to Shukaku having inherent Magnet Release curse marks, the fact that Magnet Release can be used on Gold implies that it's diamagnetic Fields, which can magnetize things that aren't inherently magnetic. Like Gold. or Sand. In Gaara Hiden, Gaara is able to use Magnet Release(with glittering metal sand) and his normal sand simultaneously with no issue.

Magnet Release is similar to Lava in that i's a lot more versatile than you'd think: In that same novel a guy uses it to absorb a bunch of iron to turn himself into a giant Lightning God.

On the Rasengan: I guess I'm seeing it as more based on it's technical functions. Its purpose is to make absolutely sure you take down the enemy, which is an important skill in a world where there are multiple ways to make yourself far more durable or much harder to kill than a normal human should be.

In addition to that, the basic Rasengan is the ultimate in chakra shape manipulation. Even if you never actually use it, simply knowing it is the kind of thing that should make it easier to learn and master other techniques. That and...

Spoiler: Was gonna be a surprise but neither of you seems interested
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Fuyuki-san, the rogue Yuki who uses his powers to make frozen confections, is meant to be a bit of an unorthodox mentor once the PCs learn his secret, in addition to Senko's source of Ice Release.

Like, he doesn't have much int he way of actual techniques r combat skills, but he knows a lot of chakra control exercises and is basically an expert in terms of Nature Transformation.

I thought it would be an amusing irony, or perhaps a call back to the Anime where the Rasengan gets used for all kinds of mundane things, if he independently created the Rasengan and combined it with Ice Release by operating backward(He started by mastering Nature transformation and then mastered shape manipulation while experimenting with the perfect way to shape that chakra to get the desired effect)... But only uses it to make the perfect snow cone. The fact that it could be used as an overwhelmingly powerful finishing move has genuinely not occurred to him.

"Alright children, none of you... Except Maybe Senko-chan if her research pans out, will be able to ever use my Ultimate Snow Cone Creation Technique, but the steps for the basic chakra shaping procedure will drastically improve your control and make it easier for you to do other things."


(Semi-Related: I didn't want to presume, but I figured a Paper Release Rasengan, based on manipulation of Ink and Paper, would end up having an effect not unlike the combination of cloth binding and a sealing tag.)