So even having a "bloodline" to create a new class seems kind of off. The story doesn't sit right with me when the story in all other cases is to produce new races, not new classes. I kind of sigh and say okay when it comes to dragons because the game *is* Dungeons & Dragons after all, so having you be a dragonblooded person who gets magic I can kind of accept. But all the other sorcerer subclasses? To me they all should be Warlock patrons.
The Warlock's story makes so much more sense and to me is so much more compelling than the sorcerer. Warlocks are the people who sit between the clerics and the wizards. Warlocks are people who have not been blessed by a god to be granted magical ability, and were not smart enough or capable enough to hack the magical system (or Weave) to pull it out themselves. So instead they go to an intermediary and make deals for it. And it is THAT class for which having 20 different types of entities granting magical power makes much more sense, and makes the stories much more compelling to my mind. A "giant soul"? It means you were in touch with a giant and were gifted a part of its power, which makes you a warlock. Not a "sorcerer". A "divine soul"? It means you were selected by a divine being who doesn't have the power and esteem of a god but still were able to grant some of your essence to a mortal, which makes the mortal a warlock, not a "sorcerer".
Good thoughts, but here's some sparse counter points...
1. It's not so much a problem of Sorcerers being superflous but perhaps of having too many stories in the game for explaining someone's spellcasting powers.
2. I don't know the final, printed versions of Shadow/Storm/Divine sorcerers, but when they were presented in UA their narrative was purposefully left vague: all of them
could be a bloodline, but they could also be something else... unlike Draconic sorcerers, the bloodline interpretation is not mandatory.
3. Not sure but it sounds like your idea of Warlock is someone granted spellcasting powers by the patron. At least the PHB talks a lot about
knowledge, not "granted powers" in the same sense as a Cleric. But the lines are always blurred (including for the Cleric), between whether the spells depend on the constant approval of an external being or only on the character.
4. Bloodline is more like those genes that can manifest after a gap of many generations. It's a lot lighter than race/species. They are of course a bit overlapping, but both concepts can coexist.
That said, as I mentioned previously, the narrative of
Giants as a source of sorcery powers just doesn't cut it at all for me. All other sources are fine... dragons are magical creatures (even tho
spellcasting dragons are marked as a "variant",
all dragons are explicitly presented as "magical" in the MM), "chaos" is nuts, the shadowfell is obviously supernatural, "storm" actually stands for elemental air, and divinity doesn't need discussing. But Giants are
not magical creatures by default, they have nothing magical, except a two of them who have (small) innate spellcasting. Obviously I am not against someone wanting them magical in their own homebrew, or a specific published setting presenting them as magical, but the problem for me is that the Giant Sorcerer
changes the default narrative of a major category of monsters in the game.