Gosh, there's room for many, isn't there? Personally, I'd like to see bloodlines that reinforce the distinctiveness of sorcerers, while allowing lots of room for different specific character origins and backgrounds. (I'd stay away from fey, fiend, celestial, and aberration bloodlines as they seem too much like other classes, in my view.)
Your astral idea is intriguing and I wonder what cool class features it might have. The Giant Soul concept has promise, too. I could also imagine an "anti-magic" bloodline that negates or controls magic around it--kind of an opposite to wild magic. Or a "destined soul" based on good fortune, coincidence, and survival that would encourage more physical if also more subtle characters. These all seem interesting to me as they feel fresh, rather then variations on familiar themes.
I like the ideas of bloodlines from particular powerful monsters/entities. It's why I like draconic so much. I'd love to see further development on the Phoenix sorcerer. I enjoy the role play aspect of such subclasses while allowing the sorcerer to slowly become more and more like the creature/entity.
I'd love to see one similar to the Storm bloodline but focused on water instead of air.
They attempted to do that with one of the Unearthed Arcana articles prior to Xanathar's Guide. The idea was to have one for each element, so the UA had Stone Sorcerer for Earth, Phoenix for Fire, and Sea for Water. I thought they were pretty neat, but apparently they didn't test too well, because none of them made it out of UA.
Big problem with the Sorcerers is their origin itself I find. I would love a blood based sorcerer, but good luck trying to shoe horn that into a neat origin story. I do like what they did with Shadow and Divine bloodlines though, I'd gladly take something in lieu of that over elemental flavor of the month.
As is, I'm still waiting for my gish sorcerer. Yeah I know Stone exists, but that's really not what I'm looking for.
If it weren't for the Circle of Dreams druid, I would say I'd like to see a fae-touched sorcerer, with a deep connection to really ancient magic. That said, I do like the idea of a planescape sorcerer who draws power from the multiverse (probably a bit too wide of a scope though).