The lack of sorcerous origins options in the PHB is one of it's very few failings. One of my players played a sorcerer with an Abyssal Bloodline in our Pathfinder campaign. That character has now been converted to 5e, and none of the options listed in the book really cut it, flavourwise, so I created an Abyssal Bloodline for use in my game.
Note: The Hellfire feature was invented by a user named Afrodyte on the EnWorld forum, so I'm not taking credit. Anyway, this may be useful to some:
Demonic Ancestor. Your demon blood strengthens your body and grants you resistance to cold, fire and lightning damage. You can speak, read and write Abyssal.
Demonic Claws. As a bonus action, you can cause claws to grow on both or either of your hands. They will persist until you choose to withdraw them. Once each round, as a bonus action, you may make a claw attack. The claws count as natural finesse weapons with a base damage of 1d4.
Hellfire. Starting at 6th level, when casting a spell that does fire damage, you can spend 1 sorcery point to add your Charisma modifier to that damage. At the same time, you can spend 2 sorcery points to force the creature hit by the spell to make a Constitution save or gain the Incapacitated condition for 1 round from the pain inflicted by intense heat of the flames.
Demonic senses. At 14th level, you can spend 5 sorcery points to gain the ability to see 120 ft. in normal and magical darkness, see invisible creatures and objects, automatically detect visual illusions and succeed on saving throws against them, and perceive the original form of a shapechanger or a creature that is transformed by magic. You can also see into the Ethereal Plain at the same range as the above.
Demonic Form. At 18th level, you gain the ability to take on your true demonic shape. Changing to demonic form counts as an action and costs 10 sorcery points.
In demonic form, your size changes to huge, and any worn equipment will be destroyed unless removed prior to the transformation. Your stats change as follows (your demonic form stats can exceed the normal 20 point limit): STR:+10 DEX:+5 CON:+10 INT:- WIS:- CHA+5. Your hit points equals 1d12 (7)+ your constitiution bonus per sorcerer level, and your armor class is 19 (natural armor). You gain resistance to bludgeoning, slashing and piercing damage from nonmagical weapons, and are immune to fire and poison damage. Your demonic wings give you a flying speed of 80 ft.
As part of your demonic form, you manifest two magical weapons – a lightning sword doing 3d8+strength slashing damage plus 3d8 lightning damage, and a flaming whip doing 2d6+strength slashing damage plus 3d6 fire damage. While in demonic form, you may make one attack with each of these weapons each round, and you are considered proficient with both weapons. You cannot cast spells while in demonic form.
Your demonic form is unstable and lasts for 1 minute, after which time you revert to human form. If you wish to change back to demonic form, you must spend 10 more sorcery points.