On Arcfire Usmash: I can't wait to be able to stick this combo-- its reliability will skyrocket.
As an aside, I think a much shorter list might be moves that cannot combo with Arcfire. Perhaps just a list on the first post of what can't, unless the specific timings and situational uses of each that can work are considered key.
Haven't managed dtilt, though ftilt and utilt work for chip damage when someone's caught close but not quite at a good smash angle/timing. Every smash, of course, works quite well, should someone be caught in 'Firewall' ranges. All aerials work, though I'm not positive of how plausible Bair would be, and to a lesser extent Dair. Jab combos seem out, though grabs, Nosferatu, and any form of Thunder can work. I'm curious as to Elwind, but I'm not sure if the spike trajectory could take any priority over Arcfire's ending knockback, even if hitting with Elwind were possible somehow.
So... perhaps 3 that can't combo out of Arcfire? Elwind/Jabs/Dtilt? Actually, in hindsight, you can certainly hit with jabs (just not the full jab combo, in my few attempts), as well as dtilt on perhaps a low % bowser or someone offstage or the like... The question's then just Elwind.