Why not let him use his motorcycle as a melee weapon and a recovery for his moveset
Great idea, BluePikmin! You know I never would have thought of that! That should totally make up for all of his Smash attacks, Final Smash, and home stage. If I've said it once I have said it a million times. Having unique abilities and weaponry will not automatically exempt you from being a semi-clone. There are characters that are just meant to share moves with others.
Leif could use magic attacks to differentiate himself from Marth. But, Sakurai wants him to share moves with Marth because of his identical body structure, design, and abilities. And so he dismissed him. He didn't make him unique because THEN he would be viable, he dismissed him as a clone before he became one.
Mach Rider too has painfully similar body structure, design, to the point where he's just Captain Falcon with a helmet and given his extremely similar body structure, design, and role, he can plausibly replicate all that of which Captain Falcon can do INCLUDING his Final Smash.
And I highly doubt Sakurai can make gun based Smash attacks.
Now I have
faith that Mach Rider could end up having an original move set. I can't base it on anything Sakurai has ever done, if anything it goes AGAINST what he's done. I can't bring evidence to promote that he won't be a semi-clone because evidence points that he WOULD be, but I believe there is a small chance he could be an exception simply because he's seen as a character who is important enough to implement to where Sakurai could tweak him as much as possible.
But, it's like a 10% chance Sakurai will add him as an original character. And a 90% chance he will dismiss him as too similar to Captain Falcon. Even his stage would be too similar and obviously his Final Smash.
Is it really worth the effort to make him into a semi-clone?
No, but that's not necessarily reason to NOT make characters into semi-clones. The bigger question is "Is it worth the effort NOT to make him a semi-clone" and while the case with Leif was no, I have faith that MAYBE, It IS worth the effort.