Whoops, forgot to respond to this sooner. There are lots of different ways ICs could be made better, but fixing some of their faults might be complicated. The biggest downsides I see are the following:
1. Nana is easy to kill
2. Popo's solo recovery is pretty poor and the synced ICs' recovery options aren't exactly great, either.
3. ICs don't cover the space above and in front of them very well.
4. Related to #3: ICs' OoS options are terribly suited for dealing with certain types of pressure.
#1 is probably the hardest thing to change since it would require dabbling with her AI: she'd need to be able air-dodge on her, up-B on her own (which would incidentally ruin a good desynch), or side-B on her own (which would mess up one desynch, but it's not a particularly good desynch and the side-B would help more). I don't know how feasible this would be, and Nana being a liability is part of what makes ICs interesting, so it's probably not one of the first things I'd look into.
Kind of like #1, #2 isn't one of the first things I'd look into changing since ICs don't necessarily deserve a better recovery.
#3 and #4, on the other hand, are things which I think could be fixed without making the character(s) feel too different. As it is, ICs struggle a lot with characters that can sort of hover around and in front of ICs or otherwise pressure them from a weird angle (Peach, Falcon, and Ganon come to mind). This might be a bit odd, but the thing I'd probably change to help with this is side-B. I think I'd make the solo version (which is more relevant OoS since you need to wait a bit after jumping OoS to do the synced version of the move) have a little horizontal boost in mobility (and perhaps a slight vertical one; I'm not sure about that and would probably lean against it), and also tweak the hitboxes near the start of the move so that they're less sporadic, e.g. make the hitboxes for the first third of a second or so hit more frequently (maybe with some GFX added to make it not look weird, unless you just change the animation of solo squall to make it fit anyways). On top of that, I'd make the last hitbox of solo squall stronger, so that it's not inherently unsafe to use as an attack, and also so that it might have some combo potential. I'd use the last hit of Brawl's synced squall as a reference point. I'd also add the stronger last hitbox to synced squall since that would have some nice applications, mainly against Peach. This incidentally could help their recovery a bit.
That's probably all I'd do as a starting point.