Doom and Gloom (subsume, subsume)
I really want to get ahead of the curve in theory crafting for the Sevagoth subsume; especially since all platforms will be getting it at the same time and the PC folks can't hog up all the ideas on their YouTube vids. ;)
I'm going to hypothesize how Gloom could dramatically affect certain warframes. As you can see in the screenie above, Gloom can be cost prohibitive (5-10 energy/sec), so we'll have to figure how to utilize better energy management than with most subsumable abilities.
All of this is on the assumption that they'll keep it as a channeled ability instead of converting it to a clickable ability.
HILDRYN - our first stop, since she uses shields instead of energy for a larger source pool to drain from and ease of replenishment. Gloom would synergize with Haven by constantly providing healing from Haven's targets. Blazing Pillage would replenish shields and health at the same time. Rakta Dark Dagger and a good sentinel setup with Shield Charger can go a long way with offsetting the combined shield drain from having both abilities toggled.
CHROMA - Rage/Hunter Adrenaline would be a must to keep the toggle active. Gloom would shore up one of Chroma's weaknesses by allowing him a way to continuously replenish his health while eating damage from external sources and the Combat Discipline strat to boost his Vex Armor buff. (replace Scream or Effigy)
(Rage/Hunter Adrenaline note: Glooms slowing effect can potentially slow the rate at which these mods replenish energy)
LAVOS - Not looking for synergies here; just pointing out the possibility that Gloom may be converted to a click power in order to function with LAVOS (which could end up being the basis of it being converted across the board).
VALKYR - Hysteria vs Gloom; in high end energy costs, Gloom beats out Hysteria by 5 energy/sec. Both provide life steal but Gloom allows you to do it with ANY weapon. Gloom doesn't damage the player on deactivation. Hysteria provides better damage mitigation. (replace Hysteria)
NEKROS - I am curious to see if his Shadows can benefit from the life steal. If not, Despoil/Equilibrium should be able to help offset channeling costs. This would be a thematic choice; as Dessication/Creeping Terrify would give you better and cost less for more flexibility in the rest of the build. (replace Soul Punch)
GARUDA - I can see a Gloom/Bloodletting combo happening here. Rejuvenation, Arcane Grace and Arcane Guardian would ensure a solid safety net for her health recovery while Rage and/or Hunter Adrenaline would do the same for her energy. I would normally suggest this as a replacement for Blood Altar but for sheer S&Gs, I may keep it just to see how insane the health regen could get.
PROTEA - she has Dispensary and AoEs. Nuff said. (replace her 4)