Take the linear sandbox concept, but make it fantasy instead of Superhero.
Allow Player development, but certain concepts be unified (magic, for instance, needs to function universally)

The basic premise is that players are sent into the 'badlands' to hunt and defeat some sort of creature or corruption. Elements of quest, but also discovery and combat, rival factions, player versus player etc. 

The Badlands, or Neutral Zone, would be territory inhospitable to the surrounding nations, preventing expansion, but also home to enchanted forests, harsh terrain and creatures which are nigh unkillable. 

Players would need to portray mercenaries, hunters etc. No character sheet, instead players would apply with an opening post. As the game would be run by posts instead of a character sheet, accepted players would be asked to produce a list of relevant names and a quick paragraph describing their character and their relevance within their plots and stories. Faceclaims would be completely option and the focus more on storytelling, text and ideally collaboration with opportunities for both large-scale group adventures and smaller dynamics. 

Dark Knights of Steel Inspired

Tattoos from bio-luminescent flora that's near invisible in the daylight but at night give the island people a rather eerie appearance.