I feel like Ken should have been an alternate for Ryu as well.
Also imagine Inklings in Smash? Could they incorporate the swimming mechanic into a platform / fighting game? Love to seem them try.
They'd have to have the swimming mechanic, no question. That's their main thing.
I think all their attacks would leave ink on the ground (except when used off stage) and then you'd be able to use certain specials only when in your ink, or their moves would be more effective when used while in ink. Though ink would probably disappear after a short while or enemies would be able to use attacks to get rid of it.
For their actual moves, they have sooooo much potential, but here's a few ideas off the top of my head:
B-Turn into squid form, can't attack, will move much faster and jump farther when in your ink. Also it might realistically be OP, but I think it'd be awesome if you could shoot ink onto the sides of a stage and then just jump into that and swim back onstage to recover.
B side-Splat Bomb (or really any kind of bomb)
B down-Ink Mine, invisible in your ink
B up-Super Jump
Fsmash-Splat Charger, Distance it goes depends on how long you charge, uncharged is 1/4 FD but fully charged covers the entire stage. Does less damage/kb the farther away you are from the target
Usmash-Splattershot, Just a powerful blast from the splattershot straight upwards
Dsmash-Splat Roller, slams the roller down facing forward so it hits both sides. Alternatively I guess it could hit only the front but also spread ink pretty far forward.
Too lazy to think of tilts and aerials but I think most of them will just be shots from the splattershot in various directions. And I guess another obvious one is one of their taunts will definitely be "Booyah!"
Regarding the Ice Climbers, I wonder if they could be reworked to fight as a single unit standing back-to-back. All of their attacks would hit on both sides, and the two would no longer be separable.
The appeal of the IC's is their unique playstyle in controlling 2 characters at once. Take that away and what's the point?
I won't speak for anyone else, but personally I'd rather they be left out of the roster than added back in without their usual play style (for the Wii U version, anyway. Don't care what they do in the 3DS version).