As mentioned - The Garai is useless for a Toa of H
Gravity, since it's less flexible and less powerful. Indeed it is made from a Toa disk of Gravity.
Similarly, there'd be elemental masks for each element made from Toa disks of the respective elements. They wouldn't have any additional powers unless the creator knew how to do so.
A hypothetical Toa Disk of Sand would make a hypothetical Mask of Sand, but both are purely hypothetical.
As for Kraahkan and Avohkii, let's break them down.
A Kanohi of Light or Shadow would be made from a Toa Disk of Light or Shadow respectively. These would be just like the Garai but different Element and would still be named Avohkii or Kraahkan, since thename represents the Kanohi power.
Takanuva's Avohkii, which there are now multiple off because of the Shadow Takanuvas abducted by Tridax, was crafted in Artakha, presumably by Artakha. It was specially crafted with the additional powers of spreading trust and understanding, as well as allowing the user to sense a beings moral light. It was also formerly infused with Toa power, which turned Takua into Takanuva.
Teridax' Kraahkan was destroyed, but the Melding universe Teridax has brought his Kraahkan into the main universe, making it the only one in existence in the main universe. It was once again,crafted in Artakha and given additional powers, allowing the user to sense moral shadow and spread fear and anger. It also would launch shadow blasts at non-Makuta who touched it, and was made from Protosteel.
Normal Kraahkan and Avohkii wouldn't have these diversions. Also, no normal Kraahkan or Avohkii exists and presumably no Toa Disk of Light or Shadow.
Where'd I learn this? By asking Greg on the LEGO Message Boards.