Companions: pick one, or let everyone choose who they'd like to do.

Colors: you did red already, so you could choose another.

Texture: assign a texture or just let everyone choose their own.

Negative space

Favorite doctor, or assign a specific doctor.

Assign a specific villain/enemy, like the Daleks, or the Master, or the Cybermen, etc.

Weather: the icons must have something weather related in them.

IDK, just spitballing here. I haven't actually participated since challenge one due to lack of time, but I'm still lurking.

ETA:

Hats (inspired by something I just saw in another fandom), or you could do scarves or some other article of clothing.