My 2cp in a nutshell:
- I wouldn't want any change to the core material, no 5.5 please. Only additions.
- As I said in the other thread, it would be nice if the Sorcerer's thematic concept of "born with magic inside" would be fully embraced, and the class was designed without access to close-form, ready-made "spells" as everybody else, but instead with access to basic powers (such as "illusion", "charm", "fire", "transmutation", "life", "teleportation", "invisibility", "summoning" etc.) and capable of using them to create flexible ad-hoc effect dynamically during the game. Unfortunately this is was too far away from the rules of magic in D&D, so it would be a major design effort, and too few people would be interested to think WotC would ever do that.
- A whole new class would be an addition that doesn't invalidate the core, but it would also compete directly with the core Sorcerer's concept. Instead, I'd be fine with a Sorcerer subclass that might explore the previous concept of flexible raw magic (presumably based on spell points, but not necessarily). But while it would be nice, it's still a fairly significant design effort as a subclass; so it might be to add new ways to represent this raw/basic approach to magical effects through new subclasses that still build on existing mechanics.
- I love the idea of Sorcerers being allowed access to any spells beyond their list. This is indeed one way to represent the fact that raw magic is inside them, and raw magic is not limited in any way.
- As an additional consideration, keep in mind that there is no more a true distinction between arcane and divine magic. There is just magic. Different spellcasters have access to different spell lists, but old ideas such as "only divine magic should be able to heal" or "only arcane magic should be able to teleport" are nonsense in 5e. The Bard is already someone with potential access to all magic, so the Sorcerer can be too.
- All these considerations do not mean that every Sorcerer should have access to every spell list. We don't need to invalidate anything core. We can simply design new Sorcerer subclasses that (1) get access to spells from other lists, just like Cleric and Druid subclasses already do, or like the Favored Soul; and (2) get additional mechanics to represent a more raw manipulation of magic, for example additional uses of the spell points pool.
- We could also always just design as many additional metamagic effects as possible, additional Sorcerer-only spells, and new Sorcerer-only feats. All these do not invalidate the PHB material, they are simply additive.