There are lots of cool characters in public domain:
Dracula
Van Hellsing
The Phantom of the Opera
Tarzan
Robin Hood
Hercules (any greek god or norse god )
Franksteins's Monster
Sherlock Holmes
King Arthur
But my favorite public domain character is the superhero "Zorro" because Antonio Banderas did an excellente job portrayng the charcater on the big screen. Honestly I think Im going to create Zorro in the game right now.