Artificer: Photologist - an artificer that has expertise in the study of photons. This sublass will have a lot or radiant damage spells and the ability to make laser traps, holograms (illusions), and laser weaponry (laser swords, laser cannons).
Barbarian: Path of the Wild Hunt - A fey/feywild themed barbarian. They will have teleportation, some druidic powers, and illusions and AoE charms will be features of this subclass. Think barbarian/ranger (hunter's mark).
Cleric: Darkness Domain - An evil/neutral domain for clerics that worship evil gods or gods of darkness. the opposite of the Light and twilight domain. It will have an ability to give temporary devils sight to party members, some necrotic spells, and even necromancy.
Druid: Circle of Shadows - A druid focused on darkness and the corruption of nature or the brutal side of nature. Nothing grows death and death is a part of nature. I'm think this druid will have a special wild shape that will allow it to turn into a shadow. Think druid/rogue
Monk: Way of the Machine - A monk that wishes to ascend to higher state via mechanical augmentation. This subclass will be similar to Prosthetist fighter and Hellion Barbarian in terms of augments. a thirds arm (extra bonus action), organ replacements (no need to breath, eat, sleep, poison resistance), arm cannon, leg replacement (double jumping distance), scanning. Heavily inspired by the Adeptus Mechanus from the Warhammer 40K universe.