Unique hostile mobs spawning in the open world 

An untapped territory in Minecraft is absolutely unique hostile mobs spawning in the open world without structures, similar to slimes spawning only in swamps, and knowing that amazing structures were added in last few years this part of the game was abandoned, and here are some ideas:

• Breezes spawn in stony peaks
• Vindicators spawn rarely in dark oak forests
• A boss/miniboss Roaming in the nether

It adds a lot to the adventure and lore, Despite the effort needed not to irritate the casual player such upgrade is needed, in the end the game needs to evolve through adding more complicated mob behaviors.