Blood Sorcery

The ability to draw magic from the inner wellspring of oneself is one that sorcerers excel at, so to do so from a literal sense is the only next logical step.

Thicker Than Water

Starting at 1st level you can use your Constitution modifier instead of Charisma as your spellcasting modifier.

Hemokinesis

Starting at 6th level you become able to tap into your natural reserves of mana through bloodletting. You can use the same number of Hitpoints as you would Sorcery Points for the same effect. Spell Slots, metamagic, font of magic, etc. 

Bloodbending

At 14th level, you become able to manipulate the blood of other creatures as well. A number of times equal to your Constitution modifier, you can cast Hold Monster without expending a spell slot. These uses reset upon a long rest.

Hemophilia

At 18th level, a number of times equal to your Constitution modifier, as a bonus action you can spend 4 sorcery points or use your hemokinesis feature in order to thin the blood of your opponent. You target one creature that you can see and they make a Constitution saving throw against your spellcasting modifier. If they succeed nothing happens. If they fail, the next time that target takes damage it is treated as if they are vulnerable to that damage type. If they are resistant to that damage type then it is treated as if they are not resistant. It has no effect if they are immune to the damage type. This resets on a long rest.