I could see a ton of potential for the ICs to be one of the most unique, fun, but also challenging characters to use in Smash--if they make it back in. In brawl they were just kinda... challenging. Honestly it seemed like they were slightly more fun in Melee. In brawl they just kinda had their somewhat useful but also risky side-B, and I guess their tether Up-B, and of course, chaingrabs. In Smash 4, I really feel like chaingrabs would be gone, and they would have a better tech-chase game and more reliable moves in general. Like, in Brawl their strength was their numbers. They didn't have much else of a gimmick. They could sorta desync and wall-off a little with Down-B and maybe Ice block spam, but they didn't really have a legitimate strength as regular smash characters.
I'd love to see that change in Smash 4, whether that change is to make them sorta heavy-weights in some way, or heavy-hitters, or relatively fast but low on damage output, or something like that. Give them a solid rush-down game, perhaps. I feel like this would be the best option, because you basically want the ICs to be the ones to engage and put themselves at risk--so that the odds of being separated are a bit higher, thus balancing them out. But they shouldn't completely focus on rush-down because that is pretty detrimental to their duo mechanic. Every offensive move they make would put their strength (being two characters) at risk.
On second thought, maybe all they need is a new forward air haha. Or a more powerful and easy-to-land spike perhaps. Then BAM. They're perfect. :3