This might be a tad lengthy because I have several alts, but I thought I would share my plans for Covenants for some of my toons and perhaps you can share yours! These are all RP reasons, not power reasons. Just my preference. And of course all this is subject to change as I play through the stories.
Draenei mage: Night Fae. Losing her family during the now infamous “crash” has weighed heavily for these last years, and the chance to possibly see their spirits will likely prompt her in this direction.
Human warlock: Revendreth. A horror-themed warlock is still looking for answers to an ancient pact on her sould. Perhaps those with power in the afterlife have some answers? (could also go Maldraxxus)
Gnome priest: Kyrian. Recent studies into the mathematical properties of “the Light” seem to point towards balance and arbitration. While not out for revenge, she may believe the Kyrian group can help shed some light on…the light?
Draenei hunter: Night Fae. Once a woodsman, always a woodsman. His bond to nature and the earth is quite strong and will likely be far more interested in an eternal forest than anywhere else.
Draenei shaman: Revendreth. Now this is a curious one, because she doesn’t easily fit into any of the groups, being a rather simple person usually. She is not a heavy fighter though, so Maldraxxus is out. Not one for justice and [redacted] way so Kyrian is not it. While she does feel a strong bond to the elemental planes, she would rather go to a party than wander in a forest, therefore Night Fae is out, leaving Revendreth.
Night Elf druid: Night Fae. Perhaps it seems too obvious, but she is following in Tyrande’s wake, outraged still at the burning of her home. A possible option is Kyrian or even Maldraxxus if they can offer better options than the Night Fae…
Night Elf demon hunter: Maldraxxus. Also perhaps too on the nose, but bred for violence seems then a natural fit in the Maldraxxus covenant. Kyrian could be an option if they push the revenge/vengeance aspect, but with the Legion already defeated she only has eyes for Sylvanas…and not in a good way…
Pandaren monk: Night Fae. A huge proponent of balance and healing energies, this likely be a good draw for her to perhaps gain some insight into mystic healing and being a conduit of life force. Second option is Kyrian but on the reserved side.
Draenei warrior: Maldraxxus. Another born and bred fighter with a hefty lust for battle. He would gladly throw down with the best of the best in the afterlife. If there’s going to be a fight somewhere, he’s going to find it and join in, and being part of the warriors of the Shadowlands seems like a good fit.
Blood Elf death knight: Revendreth. Ever since her “resurrection” she has felt nothing. But the sanguine colors of Revendreth will likely appeal to her in a deeper way, the thirst for power, the high life of the elite…memories of a former life perhaps?
Orc rogue: Maldraxxus. While not really a fighter, she is more like Garona, more of an assassin, but she would rather throw her lot in with raw power. Those in Revendreth could also sway her with their sinister plots.
Tauren paladin: Kyrian or Night Fae. This one is a real toss-up and will greatly depend on exactly how the story starts to play out. While Kyrian seems like the obvious place for paladins, he is really a nature lover from birth so the Night Fae could easily win him over, bringing his sense of justice into the wilds.