Metamagic is a mechanic that changes spells and that does not exist in the game yet.
The mechanic is new but it also require thinking about the user.interface.
Hopefully they'll have better UI options than having a separate hotbar icon for each metamagic+spell combination. Assuming Larian doesn't overhaul their UI (which they should), I'm thinking that a ~good implementation would be: one hotbar icon for each metamagic you know (so 2 for most sorcerers). Clicking it would open a pop-up window with the various metamagic versions of spells, hopefully automatically excluding any spell that can't be metamagicked (e.g., twinning Burning Hands).
However, this runs into a problem with upcasting; it'd be very messy to list all the possible metamagic spells
and their upcasted versions in that popup...
I also wonder how/if Larian will implement Empowered Spell. RAW, you use it after the damage roll, and thus after you see how many 1's you rolled. If you have to Empower a spell before the damage roll, this metamagic will become incredibly weak and ~never worth using.
