As an ex-player and GM of 3.5e, I loved the metamagic feats, and I feel bad for the fact that it isn't available in 5e. I want to make a homebrewed version of them, but before allowing my players to play with it, I want to have it reviewed, so I can rebalance it if something can break the game easily.
Here is the feat:
Prerequisite: the Spellcasting feature
Choose one Metamagic option from the sorcerer's Metamagic feature, and one of your known spells. You can apply this Metamagic option to that spell when you cast it, but the spell slot you have to expend to cast the spell is equal to the spell's level + the cost in sorcery points for the chosen Metamagic option.
You can apply this option to a cantrip, but you have to spend a spell slot of a level equal to the cost in sorcery points for the chosen Metamagic option to do so.
You still can benefit from the "casting the spell at higher level" for the spell but the expended spell slot is treated as a spell slot of its level - the cost in sorcery points for the chosen Metamagic option for the spell effect.
You can take this feat multiple times, but you have to choose a different metamagic option and/or another spell each time.
For the flavour, it has the same penalties that the metamagic feats of 3.5e have and the fact that you have to spend spell slot instead of sorcery point and that you are limited to one spell per feat make the sorcerer still interesting. It may be underpowered, but that is also why I'm here.
Here some exemples to explain some points:
Prepared spells: When you cast the spell, you can choose whether or not you cast it with the metamagic effect.
"casting at higher level": I cast a carefull fireball. For it, I have spend a 4th level spell slot. For a counterspell, it is 4th level spell. But for the actual effect of the spell (for the number of dices rolled), it is a 3rd level spell slot. If now I use a 9th level spell slot, the spell is a 9th level for counterspell, but for the actual effect of the spell (for the number of dices rolled) it is an 8th level spell slot.
Twinned spells: If you cast a twinned spell, you have to spend a spell slot that is double the actual level of the spell. e. g. I want to cast a twinned cure wound(1st level spell), I have to spend a 2nd level spell slot. If now I want to cast it at higher level, let's say 4th level for the effect, I have to spend an 8th level spell slot (4 * 2 = 8). And yes, that mean that you can't have a twinned spell that is more than 4th level.