Indeed completing missing patterns is a Herculean task! 
These last days I spent some time on part 3070b (Tile 1 x 1) as Gerald did with 98138 (Tile Round 1 x 1)
So far if I'm not wrong there are 15 remainings but keeping up-to-date is very hard compared to the number of new patterned parts being released each year.
The complex task sometimes is also finding good quality pictures, for example, I do not find a good one for this part: https://www.bricklink.com/v2/catalog/catalogitem.page?id=149890#T=C&C=110
For the priority I agree with Gerald it is not necessarily the number of sets using the part which defines the priority but rather people needs.
On my side, I started a few months ago to share photos of my own parts on a shared drive folder:
https://drive.google.com/drive/u/1/folders/1ukmHGrXcmFzgIb6RpYSti5GJZUyqZOGe
I still need to add more of them but also to remove those which have been done already.
These last days I spent some time on part 3070b (Tile 1 x 1) as Gerald did with 98138 (Tile Round 1 x 1)
So far if I'm not wrong there are 15 remainings but keeping up-to-date is very hard compared to the number of new patterned parts being released each year.
The complex task sometimes is also finding good quality pictures, for example, I do not find a good one for this part: https://www.bricklink.com/v2/catalog/catalogitem.page?id=149890#T=C&C=110
For the priority I agree with Gerald it is not necessarily the number of sets using the part which defines the priority but rather people needs.
On my side, I started a few months ago to share photos of my own parts on a shared drive folder:
https://drive.google.com/drive/u/1/folders/1ukmHGrXcmFzgIb6RpYSti5GJZUyqZOGe
I still need to add more of them but also to remove those which have been done already.