While Shadowlands is almost fully a mystery to us and we know very little on what abilities we are going to offered, what is everyone's speculative ability from the Covenants?
We already know Bastion is going to give us Vesper Totem, which increases our, or our closest ally, mastery by 20%.
Maldraxxus: Summon Graveyard Elemental - summons an elemental with tombstones and grave moss on it that does some damage and buffs one of our abilities depending on our spec. For example, a stacking buff to our next healing ability whenever the elemental does damage or give us a stacking debuff on the target that causing our Maelstrom spenders to do extra nature damage.
Ardenweald: Withering Totem - summon a totem at the location that drains the targets while reduces the cooldown of our abilities for each target damaged.
Revendreth: Blood Siphon Totem - summon a totem at the target which causes the target to take fire damage every time they attack and gives us Maelstrom and mana.