Basically anything that is outdated (ogors, Skaven, chaos demons) or something still missing (dark Aelves, Tyrion) or brand new (silent people, kurnothi, chaos dwarfs) is a great choice. Another dozen of Stormcasts is not, no matter how good the models are.
And yes I am aware of this "Stormcasts are newbie friendly" argument. It's just this solution is standard, expected, simple and far, far away from blowing minds. Unless they put Sigmar model himself in the box...
Edit: If starter boxes are about being noob painting friendly why only 1/2 of the box are Stormcasts? Neither Khorne nor Orkz are easy to paint and Nighthaunt are easy only if you have an airbrush.