Icons meaning in Awesome Kingdom: The Tower of Hateskull
After a few games, I believe we came to a conclusion about those icons...
So I'll try and answer my own question (I hope it's OK ;-))

Some of the cards with the icons are thematically connected to the Tower of Hateskull Location (Lord Hateskull, Lady Hateskull and the Stick of 6 parts to name a few). I wouldn't be surprised if the plan is to later release expansion packs for the game featuring new locations with associated cards. Those icons would then come in handy to remove all the cards from the Tower of Hateskull Theme set and replace them with the new ones, only leaving the basic ones and one scenario at a time...