Zanpaktou Abilities

[bagandscalpel]

Well, I'm sure they do have some special powers, and we just don't know yet if they are elemental or something else. Just having two swords instead of one isn't quite worth calling it shikai, I guess...

But then again, Ikkaku's shikai is "just" a spear.

[Kraco]

Yeah. But I don't expect Ikkaku to become a captain, ever. It seems pretty likely there can be even big differences in the theoretical max potential of various zanpakutou, just like the shinigami themselves are some weak, some average, and some truly powerful. So, while Ikkaku's shikai is just a spear, I expect the two captain to have something more extravagant - they likely wouldn't be captains otherwise.

Edit: A typo.