One metamagic is unlikely to significantly spoil the game, especially as you said any class can.
This means that the game will be specially designed with the possibility of casting additional spells.
There's really only 2 ways to account for player sorcerers being able to cast two leveled-spells in the same turn:
1.) Do nothing, allowing the sorcerer to start out a fight by nuking enemies and doing loads of damage/CC/battlefield control. They can do this every fight and at level 6+ - if it somehow gets back to their turn - they'll be able to do it again (because long resting is trivial). This sounds un-fun for other players.
2.) Adjust enemy HP/STs/AC to account for the sorcerer's ~doubled damage. The sorcerer won't be able to have such a drastic effect on their turn, but now other classes will do relatively less damage to enemies. This also sounds un-fun for other players. Unless Larian
also buffs all other classes, but then we enter an arms race and Larian will introduce even more (likely unbalanced) homebrew to achieve this.