The problem with Bellamy is that there's nothing he can do that others in the crew can't do. It's all about uniqueness.
I get your point, but you're speaking hyperbolically here.
@King:
Stretch punches? Luffy has that. And he does it even better in Gear 4th, where he practically rips off Bellamy by using compressed spring punches and kicks.
Bouncing? Luffy does that much better in Gear 4th to the point of flight. Sanji can literally fly too.
Seriously, when you think about it, Gear 4th was probably inspired by Bellamy himself. The whole thing is based on compressed force, which is the main idea behind Bellamy's spring powers. It makes sense, as springs are elastic devices, so it's not hard for rubber to emulate it.
And aside from Luffy and Sanji, Usopp, Franky and even Nami have their own heat-related techniques. But alas, those moves are just a small part of their moveset (including Luffy's Red Hawk). Sanji is the only truly dedicated fire user in the crew, and no one in the crew kicks like him, so there's no overlapping at all.
It's a similar argument to Zoro and Brook, who are both swordsmen, but one is a raw one while the other is essentially a magic fencer.
Depending on how you look at it, his ability can appear either similar or different.
In order to deliver a powerful punch on his adversary, Luffy stretches back his arm several meters; however, in Bellamy's case, he compresses his arm to do that. Their abilities are undeniably different from this aspect. When Bellamy's compressed punches are released, however, we see his arm stretched. Different means, similar ends (only to Luffy's Gomu Gomu Pistol though). But this is only appliable only to his (Bellamy's) Spring Death Knock.
Bellamy normally does not fight like this: He compresses his legs to launch himself on his adversary and deliver powerful, untransformed punches. Still, we hardly see Luffy's Gomu Gomu Pistol in the New World because he uses G2 most of the time: it all happens too fast that we don't see the stretch of his arm. If he doesn't use G2, he uses G3. There's a stretch, but it's still different because of the size his fists. So there would hardly be an ability overlap. Furthermore, despite their similarities in G4, they still have very different fighting styles.
In any case, it doesn't bother me whatsoever if there's a similarity in their abilities. It may feel clichéd to you, but I don't feel that way about it. If Oda decides to include him in the crew, to avoid having a technique overlap, he could be creative about Bellamy's ability.
So he could be joining Luffy as a Straw Hat, or he could not. Only time will tell. Personally, I would like that. The chapter that was dedicated to his atonement had a strong impact on me. I didn't like him in the past when he was mocking Luffy and using base, underhanded tricks, like sinking Luffy's face, while drinking his juice, into the ground in the bar & mocking his dreams, but I changed my opinion about him in that chapter & the chapter of 'I won't laugh'.