There are lots of different ways to interpret a theme or design onto icing, here are just a few of them…
Figures/models- having a regular cake (no shaping etc) with figurines on top:
A shaped cake (still quite 2d but with basic shaping):
3d (full shaped cake):
Half-relief (a flat cake with a built up design on top):
Painted (completely flat, hand-painted design):
Although you can make almost anything into any style of cake with a bit of pre-planning (and experience/skill), different things lend themselves better to different styles (and might be easier to do!). Think about what you want to achieve, what items/people you want to include and what best suits your skill set.
I’ll post some tutorials for larger cakes soon.