Some custom metamagic that I made, some of them might have balance problem, mostly because I didn't test any of them.
MetamagicYou can protect creatures from your spells full force. If the next action you use is to Cast a Spell that forces other creatures to make a saving throw you can choose a number of creatures up to your spellcasting modifier, these creatures treat the results of their saves as one degree better and are unnaffected by the spell if they get a success.
MetamagicYou manipulate the energy of your spell, causing it to converge into a single point. If the next action you use is to Cast a Spell that has an area of a burst, cone, or line and does not have a duration, the spell only affects a single creature inside the area instead. If the spell causes damage increase the damage die by one step.
MetamagicYou cast a spell that will have a delayed effect. If the next action you use is to Cast a Spell you delay it to be used later for the next minute by spending one concentrate action on your turn. You must make any decisions for the spell when you first cast the spell, such choosing the area, targets or effect of the spell. You can have as many delayed spells as you like, but only one can be activated with a single action.
MetamagicYou can redirect the energy of a failed cantrip into another. If your next action is to cast a single target cantrip against a creature, and you miss the foe with the spell attack roll, you can cast another cantrip of 2 actions or less on the same creature as a free action.
MetamagicIf the next action you use is to Cast a Spell that targets one creature, spend another spell slot or prepared spell of the same level to target a second creature in range.
Twinned is the one that I am pretty sure that have balance problems...
Metamagic
Prerequisites Signature spell class feature
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By take an extra moment you can bend the strength of your spells at will. If the next action you use is to Cast a Spell you can heighten the spell at will like a signature spell.