Given that Clerics already have 4 DPS souls, 3 Healing souls, and 1 Tanking soul, my guess is that the new soul for Storm legion will be a Support buffing soul similar to Bard & Archon. A Support soul would fit in very well with the Cleric's other souls and hasn't been implemented yet.
I kind of like season themes in games (Realm of the Fae, Click Clock Wood in Banjo kazooie), and I've always felt there should be a solid class based around them. There's a free MMO called Sacred Seasons 2 that uses this for each class but it's kinda a clunky game overall.
So I was thinking a great idea for a new Support Cleric soul would be a soul that communes with the four seasons (Spring, Summer, Autumn, Winter). The overall gameplay style would probably be pretty similar to a Bard or Archon, however the buffs you use have a different general effect depending on the season they belong to.
I would have there be at least one type of buff that empowers your spells themselves, for instance:
(only one Empowerment can be active at any one time per Cleric)
Spring Empowerment - Empowers the Cleric with the life of Spring, giving spells & attacks a chance to heal a nearby target for x% of damage or healing dealt.
Summer Empowerment - Empowers the Cleric with the blazing sun of Summer, giving offensive spells & attacks a chance to deal an additional x Fire damage.
Autumn Empowerment - Empowers the Cleric with the changes of Autumn, giving all spells & attacks a chance to grant x haste to all party and raid members for 10 sec.
Winter Empowerment - Empowers the Cleric with the freezing snow of Winter, giving offensive spells and attacks a chance to reduce the target's movement speed by 50% for 3 sec.
You'd have smaller buffs & spells of all seasonal types similar to Bard songs the more points you put into the soul, but those would be the basic general effects of each based on the season (Spring - healing, Summer - damage, Autumn - buffs, Winter - control).
And then at 51 (61) points (or the top tier of the point tree) there'd probably be an ability like this:
Unleash Seasons - Unleashes the power of the seasons, causing a different effect based on your currently active Empowerment:
Spring: Heals targets within 30m for X
Summer: Deals x Fire damage to the target and all targets within 7m of the target, and increases the damage taken by those targets for 15 sec
Autumn: Increases the Haste & Crit chance of all party members by x% for 15 sec.
Winter: Freezes all enemies within 10m of the Cleric. Damage caused may interrupt the effect.
So granted, this would give you a lot of flexibility as to what "type" of Support you need or want to be. For instance, you can use Spring for fights that need extra healing, Winter for Warfronts, and so on.
I would also personally make it so that you can get the most basic buffs & Empowerments with no more than 20 points spent so it would have good synergy with the other Cleric souls (51pt Healers taking it for the basic Spring healing buffs, Inquisitors getting it for Summer Empowerment, etc.)