So a series about a young man/boy improving his abilities to protect his "family"/friends and using the concept of inherited will (In Reborn's case, the Dying Will, One Piece's case, the Will of the D), as well as becoming the best (Mafia Boss, Pirate King) is nothing like One Piece? Okay, then.
The story may not be the same, but there are a lot of shared themes.
Dying Will is nothing inherited. In Reborn, to do something with a Dying Will is to do something as if it's the last thing you're going to do – the Dying Will Bullet didn't work on Ryohei, for example, because he always does things to the extreme.
That's nothing like an "inherited will." You can't even compare it to One Piece's Will of D because no one knows what the Will of D even means. Tsuna doesn't even want to be the Mafia Boss – the Reborn series isn't about him wanting to become the best -- it's about him wanting to protect his "family."
Sure there are plenty of shared themes in shounen. That's what makes them shounen. But to say that because they are shounen they are similar is false. Otherwise, what would be the difference in reading One Piece, Naruto, Bleach, D.Grayman, Eyeshield 21, JoJo, Reborn... and so on?
They have a few things in common, and of course you can make comparisons if you use a sentence to simplify things (I can compare Hilter to George W. Bush, for example, but that doesn't make them alike)... but you can't convince me that One Piece and Reborn are similar series. They read, progress, and go about telling their respective stories completely differently. To compare them because both of them have terms that include "will" in them is to make a weak arguement, imho.