Objects and faces, that's great! I didn't know you were going to make faces as well, so in the meantime I had to cut the face of the characters and use those, but it's not the same as with proper, more detailed faces ahah xD 
A castle or a dungeon, they both are useful... well of course it depends on what you want to do first, but thinking about my game, since you already made this cave pack, maybe a castle? My idea for this game is an interactive adventure with exploration and dialogue choices, so a castle to explore would be interesting.
Of course your game comes first, don't worry!