This month's Unearthed Arcana (they are on the second Monday of every month now) deals with "fiendish options". "In time for the lengthening shadows of autumn, we present a collection of fiendish options for you to playtest: new subraces for tieflings, ways to customize diabolical cults, and demonic boons that wicked NPCs can receive from the denizens of the Abyss."
Demonic boons? Mostly benefits with no downside (Zuggtmoy and Jubilex aside)? Well, that is interesting. I think they need to have a downside that comes into play when the boon is taken away, and perhaps an 'upkeep' requirement (sacrifices, etc...) to keep the booned entity loyal to their master.
The Tieflings are fine, but the more versatility you offer, the less reasons there are to have anything specific in place. Why not make it: You get +2 to Charisma and +1 to an ability of your choice. You gain an appropriate cantrip at 1st level, a 1st level spell you can cast as a 2nd level spell at 3rd level, and a 2nd level spell you can cast as a 3rd level spell at 5th level, but they must be spells appropriate to the Diabolic entity from which you are descended.