Google announced a bunch of updates to Material Guidelines during io2018. One of the updates is themes of material icons - https://material.io/tools/icons/
It now has "Filled", "Outlined", "Rounded", "Two-Tone", and "Sharp".

In Flutter, I know how to use regular icons which are "Filled" ones. I also know how I can add individual assets to the assets folder in Flutter app and use those in my code but I'd like if they were readily available so that you could use them like -

IconButton(
  icon: Icon(Icons.volume_up), // <=== Point of interest here
  tooltip: 'Increase volume by 10%',
  onPressed: () { setState(() { _volume *= 1.1; }); },
)

Looking at this repo I think it only supports default (Filled) right now but are there any plans to add other themes?

Probably

IconButton(
  icon: Icon(Icons.volume_up, theme: IconTheme.outlined), // <=== Example
  tooltip: 'Increase volume by 10%',
  onPressed: () { setState(() { _volume *= 1.1; }); },
)