I'm not opposed to that. In fact, double-casting was my original intention (because Action Surge clearly establishes that double casting is valuable but not game-breaking), and it wasn't until I actually wrote out the fluff text that I decided it sounded more like double concentration than double casting. But really I don't have a strong opinion either way.
I think capstones and near-capstones are important primarily as temptations/aspirations. The fact that a Moon Druid can become an Onion Druid is an excellent reason not to multiclass a level or two of Rogue or Barbarian! Likewise, it's really painful as a Bard to take more than two levels of any other class, because then you lose all hope of Wish. The Fighter capstone is excellent too (IMO), but you can make up the damage by multiclassing Rogue, so Fighter 11/Swashbuckler 9 is a viable competitor IMO to Fighter 20. Ideally, I'd like all the capstones in the game to be good enough that it's always a painful dilemma to multiclass at all.
Anyway, if you're going to do something as foundational as "let the sorcerer pick from other spell lists," I'd actually rather build that into the base sorcerer chassis as a variant rule than into a subclass. It just doesn't feel like a subclass thing to me--it feels like a statement about sorcerers in general, which perhaps ought to be its own thread, but I'll throw out an idea anyway:
What if each sorcerer picks a school of magic as his focus, and is then able to learn spells freely from that school, from any class list? BTW that would interact poorly with the proposal in this thread--I would definitely rewrite Instinctive Magic at any table where this were a thing.
That makes the Sorcerer kind of the Warlock in that it would have two separate decision points: origin and focus, akin to the warlock's pact and boon. The intent would be to open up sorcerer versatility wide open ("there's nothing that a sorcerer can't do, in principle") while still keeping actual restrictions on PCs tight enough to avoid overshadowing bards and wizards. Any given sorcerer is still only going to have maybe 25% of the spells they'd really like to have on their list, but you no longer have to answer the question "why is summoning with sorcery inexplicably not a thing?"
Well, I have some stories and in them I like to describe the way things work, like in one there is a power that is explicitly the full opposite of magic and they interact in strange ways, or in other certain creatures are basically immature angels, ... but I'm babbling. So in short:
In stories focusing on witches, each witch is unique with magic being a reflection of her personality. But they can be roughly grouped according to the things they normally do (I have used these guidelines to help players pick thematic stuff for a campaign, just short of actually writing the speciffic spell lists):
Attuners: Masters of a single element/thing. Cannot do much with things not related to their element, but they can do almost anything with their element (Metal, wood, fabric, fire, water, toys, books, whatever)
Dreamers: They are masters of the ethereal stuff, be it dreams, thoughts, illusions, visions and stuff. But have a hard time affecting physical things
Shapers/manifesters: They create and manifest things, from illusions, spirits to physical items -but not necessarily mundane items- and creatures. Controlling and breaking stuff is another story.
Controllers: They won't create anything ex-nihilo, but can manipulate and bend whatever thing they wish
Warpers: They basically transmute, alter, warp, crush mater and space.
Each of these things is basically a different spell list, maybe attuners is many similar spell lists instead. The advantage to schools is that schools are explicative but not necessarily thematic -and we don't need yet another way the sorcerer is like a wizard mini-. This way we also have more balanced lists that allow certain flexibility.
For example Controllers will know certain transmutation spells related to manipulation while warpers those related to transform stuff but won''t know the enchantment spells that dreamers and controllers do. in turn dreamers can do illusions just like shapers, but not summonings (which they will have in common with warpers.) Attuners on the other hand can do basically anything as long as it is related to their attuned element.
