@SGRaaize:
But we won't ever be able to discuss that until we see it happen, you can't make up things the Devil Fruits can do and discuss them seriously.
Despite Oda having the real say on what DF's can and cannot do, discussion is our freedom of expression in action. Yes we can discuss.
Besides, there is a real-life contradiction in your statement. Math. Math is really entirely conceptual, and has always been. Due to its conceptual nature people criticize it for not being applicable to reality. They were dead wrong. Now everything that is measurable can have math used on it, and the calculations are accurate.
However, this is not math, because absolute logic isn't applied to the concept of the Devil Fruit. In fact, the name implies a refusal of logic. So is it still discussion worthy? Yes, because it was a product of imagination in the first place, so applying more imagination to it is viable. And the nature of imagination is flexibility and fluidity. It doesn't stay put in one place, so why should its products?
Anyway, why does Bellamy only turn his legs into springs? And why legs in the first place? Legs are the primary means of propulsion in humans, so that's the natural starting point. He never got passed that.
Even Doflamingo got passed using strings to pull things with. (how else would he be a Shichibukai)
The point of my "thought experiment" was to delimit the fruit's usage from only what's shown.
For example, Luffy's usage of his DF moved, from stretching and snapping back, to increased power output in Gear 2nd and increased mass in Gear 3rd.
However, going back to the original idea of categorizing DFs into tiers or power levels. It doesn't work honestly. There's really nothing holding back the power other than the user's own physical and mental limits. If there was an indestructible character with unlimited physical force using Gomu powers, that character, not affected by physical stress, would be able to stretch his arms into space, grab the moon, and slam in onto the ground. Gura Gura no Mi too overpowered? Not compared the the Gomu Gomu. Also, due to the character being indestructible, he can use Gear 3rd (due to the limited quantity of air on a planet, this character can use the Sun's plasma instead) to create fists bigger than the planet and smash the planet between them. He can also give Galactus (Marvel universe) a black eye.
In conclusion, it's not the fruit that has power, it's the person using them. The fruit just gives new abilities. Heck with prejudice, if the Hiso Hiso fruit (Apis's DF) were canon, in the right character, it may be able to read the minds of all creatures on the planet, and possibly control them too, like Professor X in X-men.