2) Remove the sorcery point cost for using metamagic - With the exception of twinning high-end spells, or using a high-end spell that does repeatable damage and quickening others, the most I can see happening is double Firebolt becoming popular, but that still does less damage than (Agonizing) Eldritch Blast with Hex.
At lower levels, this is still feels extremely powerful though.
I can
always cast without ever alerting anyone to my doing so and I can't be stopped with a
Silence spell or being restrained?
I'm
always giving Disadvantage to enemy saves on spells like
Blindness/Deafness or
Hold Person? Granted, it's only on the first save, but even the Blind condition for 1 round basically ends the fight once the rest of the party jumps in.
I can Quicken
any spell forever, basically meaning I'm always casting as a Bonus Action because why wouldn't I unless I'm converting SP to Spell Slots? Also meaning that I can always follow up with a Cantrip on top of that, which by the time I'm high enough level to get the extra dice means it's going to be Empowered
Fire Bolt,
Ray of Frost, and
Shocking Grasp to just stack on the damage. Or I can even Quicken
True Strike and then make sure my follow up attack Cantrip is basically always hitting, which accuracy is a big part of keeping 5E balanced.
It's not really the High-End spells I would be worried by, it would be lower-end play where a 3rd Level Sorcerer now can cast Empowered
Scorching Ray and Heightened
Web at will, or always follow their Quickened Whatever with a damaging cantrip right behind it.
I'd have to think on it more, but at first glance this seems to really heavily tip things towards keeping the Sorcerer a blaster, but one that has a howitzer instead of a rifle, especially if you eventually get all the MetaMagic options anyway, so the inclination would be to take the most powerful ones out the door, and then every Sorcerer ends up the same in the end anyway, with Empowered, Quickened
Fireballs followed by Empowered
Fire Bolts.
I like the idea of making Metamagic more flexible and useable: it's a key feature of the Class and one that could use some expansion, but I also think there needs to be a balance between Sorcery Point spending, Honestly, if the Sorcerer just got more Sorcery Points in general per level, that helps them flex that Metamagic muscle while still not being
overly concerned that they'll run out and not be able to conjure up another Spell Slot, but it still keeps it where they need to balance between either More Spells or Better Spells.
Also, just some more Metamagic options in General. Maximized Spell is sorely lacking and would be an awesome nova option for when the enemy really needs to just go away. Or what about Enlarge Spell, making AoEs affect a wider area for when you really just need to put 20 kobolds to Sleep? Or Intensify Spell to cast at a Spell Slot Level you don't actually have yet?
Your first point, restricting metamagic to only Sorcerer spells, I'm generally good with. I'd wish their spell list was expanded to help with this, but I understand the necessity to reign in some crazy MC that starts really unbalancing the Metamagic system. And again, the 3rd Point, I'm not sure on as it means every Sorcerer ends up the same, with all the options on the table. If there were more Metamagic options added, however, this would be a great bonus to help design a very flexible, but still unique playstyle.