Wisps, in popular culture, refers to spherical thunderbolts as well as to a kind of dwarf or fairy.
In minecraft, wisp would be a new flying mob of humanoid appearance, but a very reduced size (like vex) or may be a simple little flying cube; and with a surrounding halo or aura of light. of cyan, yellow, orange or purple colour, depending of the type of wisp.
Wisps will spread effects or affect to near mobs, while they are flying. After die, these can create big zone effects or modifications, etc. These mobs will try to do "kamikaze" attacks against others specific mobs or after interact with these (I will explain this better more down), or simply these will die by random values.
Wisps would have a low life of five hearts or less, but these will create the same area damage/effect when killed.
Types of wisps:
For now I only suggest two wisp types, although originally I thought in five types.
Wisps of yellowish colour/aura that spawns during or after storms in any biome, and can go through alpha blocks. When dies, this mob will summon a thunderbolt.
As all the others wisps, this mob can die randomly, summoning such thunderbolt.
Usually this wisp type doesn't go too near to others mobs, but this will do quick mini-electric discharges to any mob (players also) that it gets too close, with a damage of half hearts or no damage.
Electric wisps will tend to "attack" (kamikaze) against metallic blocks (redstone, iron and gold blocks, and iron bars (not redstone mechanisms)), and against creepers (facilitating the obtaining of charged creepers).
This wisp also will attack (move, die and summon lightning) to the player, if this player attacks to the wisp.
Ice wisps (of cyanish colour/aura) will spawn at cold biomes, during or after storms (unlikely during or after snowfall), and can go through snow, water and ice blocks. When dies, this mob will spawn a block of compact ice surrounded of normal ice, and give slowness level I during 5 seconds to any mob inside a radius of 5 blocks zone, and create snow over the blocks of this 5 blocks radius zone.
This mob will not be as evasive as common wisps, and the chance of this for die randomly would be minor. When this mob fly near to others mobs (including players) will give slowness.
This wisp will tend to die far (but enough near, inside radius of action) to others mobs, or attack to the player/any other mob, if this player or mob attacks to the wisp.
Both will have low spawn rates, and the spawning of these mobs will be conditioned by storms.