A game where you start out as a minor demon in Hell (or other Underworld counterpart) and you raise in the ranks to become the very King/Queen of Demons him/herself.
Now, I know there were a few games, but here's my vision of the otherworldly structure.
NOTE: While this one is sorta build around the MLP universe (to at least some extent, not necessarily story-wise), it can be quite easily implemented in almost any other universe, new, old or fan.
When one dies (in somewhat unspecified game world), he is sent to a parallel dimension, known as the Underlands, which mirros the "main" dimension of the game - why is it important I'll talk about later.
Such a newcomer (for lack of a better word) is known as Alpha Demon. (even if he doesn't commit any demonic deeds for the rest of eternity)
There are 24 ranks of demons in the Underlands - from Alpha to Omega, named after the letters of the Greek alphabets.
Alpha Demons are devoid of any powers they might have had when they were mortal - a unicorn would have his horn broken/shattered, a wizard would lose his magical powers, a warrior would no longer have his legendary strength.
To regain their lost power and to gain new powers, the demons must consume souls of the living - (even if the demons themselves technically ARE souls of the living, kind of) - and there are two ways to do it.
Serving the mortals - the game would offer a HUGE amount of quests, in which the living summon the demons, sacrificing other living creatures to offer their souls to the demons for whatever they might wish to do. They would vary from stealing an artifact sword from the king's castle to murdering the king in question. (and claiming his soul as well)
Defeating other demons - the Underlands are a very unfriendly place, with demons constantly fighting for power with each other - whoever challenges another demon to the battle and wins claims all of his souls, and as such, his powers. (which will cause the defeated to become an alpha demon). Defeated demons serve as minions to their master, and all the souls that they would lay claim for will instead be awarded to the master (the "debt" is equivalent to the amount of souls the minion had before being defeated - this means that alpha demons are safe from the "enforced labor", but are also devoid of power).
Captured souls are reflected in one's hair, turning it red - one soul equals one red hair.
Said souls also serve as the game's experience points and "skill" points, although I'm not entirely sure how the system would work. It would be heavily dependent on what universe the game would settle in.
Demons can only enter the world of the living when summoned by the mortals at the locations in which the mortal in question is trying to summon them.
Might sound confusing? Let me give you a somewhat vague example to clean the confusion:
A new alpha demon has entered the Underlands, who we shall call Israfil (not to be confused with Israphel). Israfil was an alicorn/winged unicorn in life, but now his horn is broken and his wings are ripped off.
As he died in the forest in the mortal world, he regains consciousness in the said location - which looks fairly similar, but much less lively - burnt trees, red sky, an ominous fog...
After entering an undead version of a village, he senses someone is trying to summon a demon in a nearby house, with an offer of 1 soul.
No other demon seems to notice that, so Israfil decides to enter (ie. walk through the wall) of the house and sees a white pentagram (or other symbol) on the ground.
As he enters it, he is teleported away from the Underlands, back into the living world, still in his black and white form.
His summoner stands mildly shocked that someone heard his prayers and asks Israfil to kill, a bandit leader who has been threatening the village. (you know, not only the bad guys summon the demons)
After the leader being killed (with little struggle, since all demons apparently are spectral beings and immune to physical damage), Israfil claims his soul and the soul that was used in the sacrifice to summon him, therefore gaining two souls, which allows him to regrow his wings and the magical horn, becoming the beta demon.
Way longer into the game (borderline end-game), Israfil manages to defeat the current omega demon (the most powerful demon in the Underlands), which not only grants him millions of souls that the omega demon has accumulated thorough the centuries, but also all the souls of his minions, and minions of his minions. On top of that, the omega demon is degraded to a mere alpha demon and has to work the thousands of souls before being let free (as are all of his own minions).
Immense power now belongs to Israfil, but no mere mortal might summon him - he can and will ignore the offers of a single souls, and more will be needed.
Not only that, but Israfil also chose to reside in a faraway and dangerous location, sealed off to the mortals.
If that could be just a PART of another game, the mortals that summon the demons could actually be other players, which would spice up the gameplay a great deal. (assuming that the demon you summoned didn't decided to just kill you and claim your soul as well, but given that you are in control of it, it probably wouldn't)