Characters I have the biggest questions about the viability of are ICs, Palutena, and Bowser Jr.
ICs just have a lot of complicated unknowns. Their damage looked great, but there are many links in the chain that could break for them.
Palutena as she exists in Smash 4 benefits very little from the systematic changes, probably the least of all characters. Doing stuff out of a run doesn't help her much, despite her run speed. 3f jumpsquat isn't a huge change from what she had. Landing lag wasn't her limiting factor. Air dodge works against her uair. Her specials still seems poor. We'll just have to see what improvements her core moves got to keep up.
Bowser Jr. also doesn't care about doing stuff out of a dash, though like Charizard he probably loves the new jumpsquat more than anyone, even his dad. He was just such a rigid character before, with what I felt was maybe the lowest ceiling in Smash 4. But Zelda suffered the same problem, and got a revolution. So we'll see!