Theory: Investiture cycles and Harmony

[Fallen Rope he/him]

Any shard can power any investiture but at a cost.

CZANOS (17 OCTOBER 2008)

Preservation can fuel Allomancy, (Minus Atium.) but can Ruin fuel Hemalurgy? (Or Atium?) And could Sazed fuel all three Metallic Arts?

BRANDON SANDERSON (17 OCTOBER 2008)

Both gods could, if they wanted, fuel all of the metallic arts. Preservation is stronger at fueling Allomancy, Ruin stronger at fueling Allomancy or Feruchemy when it has been given via a spike. Both are balanced when it comes to Feruchemy. But this rarely comes up in the books, as it required expending power in a way that the gods were hesitant to do.

When Preservation powers Allomancy the power returns to the shard after it has been lost. So if Ruin powered Allomancy the cost could be that the power returns to preservation instead.

This could be how Harmony is using the extra ruin he has. He’s powering Allomancy with a bit of ruin and preservation,  same for Feruchemy.

Since Allomancy is being powered in a more balanced way it makes the barrier for snapping lower.  The extra ruin might be why there are no more mistborns or full feruchemists.

[Moogle]

I'm not sure that Presevation "powered" Allomancy in the way that you're suggesting. I think that WoB refers directly to what Vin did with Elend.
 
All the same, your theory has some pretty strong footing. Snapping is required to overcome the little bit of Ruin inside of Scadrians. From the Annotations:

My idea on this is that Allomantic potential is a little like a supersaturated solution. You can suspend a great deal of something like sugar in a liquid when it is hot, then cool it down and the sugar remains suspended. Drop one bit of sugar in there as a catalyst, however, and the rest will fall out as a precipitate.
Allomancy is the same. It’s in there, but it takes a reaction—in this case, physical anguish—to trigger it and bring it out. That’s because the Allomantic power comes from the extra bit of Preservation inside of humans, that same extra bit that gives us free will. This bit is trapped between the opposing forces of Preservation and Ruin, and to come out and allow it the power to access metals and draw forth energy, it needs to fight its way through the piece of Ruin that is also there inside.
As has been established, Ruin’s control over creatures—and, indeed, an Allomancer’s control over them—grows weaker when that creature is going through some extreme emotions. (Like the koloss blood frenzy.) This has to do with the relationship between the Cognitive Realm, the Physical Realm, and the Spiritual Realm—of which I don’t have time to speak right now.

 
If Allomancy were to somehow use Ruin more, there would be less of a conflict needed and this might lower the Snapping threshold. I don't think it's possible for Allomancy to use Ruin like that though. Maybe? There's two types of mist, presumably Preservation/Ruin mist, and being in either might increase your Allomantic power which might imply Ruin is powering some Allomancy. WoB:

Chaos (15 October 2008)
Do all three Metallic Arts still exist after the events of the book? Are Allomancy and Hemalurgy slightly degenerated now that Ruin and Preservation are dead, or does Allomancy still draw upon Preservation's power (just held with Sazed now)?

Brandon Sanderson (15 October 2008)
Allomancy, Feruchemy, and Hemalurgy all work as they once did. However, now they are more directly affected by the presence or absence of the mists, which will slowly return to the world but not be of the extent they once were. (The mists are now an extent of Sazed's power, and where they roam, he is better able to influence things. There will also be two kinds of mists.)

(source)

Definitely something to think about.