I want my HA to be simple, but it feels too plain. What could I add to it without it being too overcrowded?

Some kind of a simple belt. I would suggest the pale red big buckle belt [Item=Pale Red Big-buckle Belt]

I would add some jewelry, a bracelet and an anklet maybe, to go with your yellow neck bow. The face area also looks very plain, try adding some blush or a nice pair of lips. A hair accessory might not be too bad either.

Thanks for your suggestions. :) I'll definitely try them.

a simple pair of earrings and ring would do wonders. I agree about the blush as well, it will give it a nice tone. Or perhaps a pale pink lipgloss?