Joseph--they don't have any magnet people really, at least not at his power
Nate Grey--pure power. They have a telepath with MM, but not at this power, where Nate can rip images from people's heads and make telekinetic images of them.
I agree with Iceman, Magma and Storm, elemental powers which the JLA are somewhat lacking in.
Colossus, I think, may actually have a shot. Sometimes, you just need a strongman, the warrior-artist, and one hell of a handy-man in tough situations. They've let other less powerful people on the team. Green Arrow is just a Batman with arrows. Hawkman and Hawkgirl just fly.
Probably Forge too, his technomagery would come in hella handy.
They already have steel, who is a kind of colossus look-alike
But Steel is more of a weak Iron Man ripoff.
As for Colossus, I wouldn't put him on the JLA team. Not on the "big 7" with Supes, WW, Batman, etc.
But on a team like Green Arrow, Hawkman, Black Canary, Flash, Green Lantern and Elongated man, they're missing some serious muscle. That's what he'd be good for. And he's solid steel. Once in awhile something will be able to affect Supes or WW's biological aspects, but it can't hurt steel.
I could see Wolverine on a black ops team, like JL Elite or something. Though with his age, he might actually fit in better at the JSA.
As for having elementals, some of the members will be vulnerable to temperature and elemental extremes. Someone made of ice, or magma, won't be affected, and can operate freely, as well control said elements. It's just a new power aspect that they're somewhat lacking in.