My wife and I started watching Diablero, and while she is writing up what the show is about, I am writing up how I would setup a game using this setting. You can find more about the show here, and you can watch it on Netflix.
As of the end of Episode 2, there still wasn’t much information on the breadth of the powers that the Diableros have. So far we have only seen an Exorcism power used by both Issac and Elvis, though Issac seems to be able to use the power from a distance while Elvis seems to need to be closer to his target. This might have something to do with Elvis not having anyone on hand to trap the demon while Issac had a minion do that part while expelled the demon from its host.
As for the possessed, they seem to gain some amount of toughness, agility, and strength while possessed, but they also seem to gain a vulnerability or weakness to electricity. So far I might model this after a species adjustment of +1 dice to two of the three attributes (Strength, Vigor, or Agility) and a Attribute Penalty of -2 to Smarts and an Environmental Weakness to Electricity. Though this is still subject to change based on what other things we learn during the show.
Another reason that I am leaning towards using Savage Worlds for this is they have a mechanic for interludes which can be used to fill out characters back story like this episode did. The mechanics for an interlude are each of the players draws a card from a regular deck and depending on the suit of the card (and if the Interlude is for downtime, back story, or travel) you get a prompt for the player to tell a short story based on the prompt. In this episode, it seemed like we got a bit of back story from the point of view of both Elvis and Nancy covering how they met, and how he helped her learn to control her demon.
On a final note, Savage Worlds is not the only system that I can see running this game in, but it is my preferred system. The other one that I can easily see doing this with is Fate or Fate Accelerated edition by Evil Hat Productions.