The best replacement for royal icing is buttercream.

The best alternatives to royal icing are rated as following:
★★★★★  buttercream
★★★  fondant
★★★  color flow
★★  buttercream icing
★  glaze
★  icing
★  cookie icing
★  sugar glaze
★  icing sugar
★  buttercream frosting
  gum paste
  fondant icing
  sugar paste
  gumpaste
  marzipan
  meringue powder
  glaze icing
  corn syrup icing
  boiled icing
  butter cream

Your alternative for royal icing is not listed? Suggest!
Speaking about royal icing, one probably shouldn't forget about meringue powder.

Other search terms related to royal icing and their ratings are:
★★★★★ meringue powder
You have in mind a companion for royal icing and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of royal icing in all years. Click on graph for more.
royal icing
buttercream
fondant