This is basically the premise of the Shadow Council. Powerful demons affiliated with the Burning Legion are in charge and they bestow power upon promising mortals who sacrifice souls as pacts.
(See Shadow Lord Fel’dan in this example: https://wowpedia.fandom.com/wiki/Shadow_Lord_Fel%27dan)
Gayagenda (can’t believe you made me type that) above also names some proper examples and raises a good point. However, much lore from Vanilla and TBC has since been retconned and is best viewed with a little scepticism.
Overall, I believe that only powerful demons have knowledge or power to bestow. Mannoroth’s blood empowered the orcs because Kil’jaeden wanted his army. Varimathras plot to take over the Undercity and do whatever Legion magic he was up involved recruiting Grand Apothecary Putress as his underling.
In extremer cases, we see mortal races empowered so by their demonic masters that it alters their form.
- Fel Orcs get a red skin in TBC, but in WoD we see orcs twisted so that bone-like spikes grow from their flesh, horns can grow from their head and their hands are sometimes turned into blackened claws due to the fel energies.
- Draenei aka Eredar get either a red skin or dark skin, their horns twist and fel energies can be seen piercing their skin and blackening their hands. Some Eredar are shown to have wings.
- Blood Elves can be seen growing horns, demonic tattoos and teensy little wings. We also see some fel-gobbling Wretched.
- Highborne/Kaldorei/Nightborne/Naga Generally they gain hands glowing with fel energies. There is a risk they may grow even more smug.
Usually, a character bestowed a great deal of demonic energy causes their skin to crack, alter their eyes (red/green/Voidish), sometimes grow horns, hooves and wings. Another side effect of being ‘gifted’ to the brim is an increase in size. This being especially true for the orcs and Eredar. I didn’t include demon hunters because if you took the time reading through this forum there’s a good chance you know what they are.
In unique examples, creatures are twisted altogether. We see Gul’dan transform into a massive, hulking brute during the fight atop of the Nightwell. Teron’gor becomes a bloated abomination (basically a big mouth) after gorging on souls in Auchindon. Xavius was given an entirely new body by Sargeras himself, giving rise to the first Satyr.
I personally think this is done to signify their level of corruption and to make them stand out to the player. Or maybe just for the cool factor.
I will also touch upon another ‘blessing’ demonic powers can bestow. Reanimation and undeath. We are given a spare few examples:
- Kael’thas Sunstrides is resurrected by the Shivarra Delrissa. We don’t know how exactly, but it’s probably got something to do with that big stone sticking out of his chest. Merely a setback, surely.
- Madrigosa aka Felmyst, a blue dragon whose bones are reanimated after the blood from the Pit Lord Brutallus seeps from his corpse into the ground beneath her.
- Soulstones, an ability granted to the player. We see in the most recent warlock quests that this can tether a soul and reanimate someone. We aren’t really told how it works and generally I see it much like how Whitemane had the ability to resurrect the Scarlet Crusaders back in the day. It just works.
- The Risen, former Scarlet Crusaders in Stratholme (and likely in Tyr’s Hand) who were killed and reanimated by the dreadlord Balnazzar. This is probably just necromancy, but since this is pre-Shadowlands lore and it involves a demon raising the dead it is something to keep in mind.
Overall, I believe we simply aren’t given enough details on how reanimation by fel works specifically. It may just be a leftover from the era where Blizzard hadn’t decided what direction to take. It could be done, but give it some proper thought and let it remain a rarity.
In conclusion:
Yes, there are demons who may consider generously adopting a mortal for their own needs. Especially since the Burning Legion suffered a heavy blow at the conclusion of the Legion expansion and there may be a struggle for power. I think this needs to be a potentially powerful demon and the mortal needs to be useful (up to a point, anyhow).
Unlike in DnD where I believe pacts are generally a two-way trade, the Warcraft demons are ultimately just as self-serving as many warlocks and your warlock will end up a servant themselves. No wise demon would grant a mortal any power that can be used against them.
Lesser demons will likely just try to eat your face and are best kept on a leash. Though they could try to deceive your warlock, which may well make for some fun RP!
As I was writing this, there have been some replies to this thread and what is brought up may overlap with the above points.