Generally filters meant specifically to help discovering new titles someone might be interested in.
Examples off the top of my head:
1. Newly updated manga that have had less than N chapters posted so far.
2. Newly updated manga that before the last update had not been updated in T time.
3...
This's not specific to MangaDex, but it's relevant given the current system of limited tags and how additions and edits are doable only though staff.
1. What's a genre? What's a theme?
2. How strongly should a genre or a theme feature in a work before they're added as tags?
3. What makes a...
Currently, sorting by "Rating [UP]" sorts the higher rated titles _first_, while "Rating [DOWN]" does the opposite.
Treating scores as ordinary numbers and sorting accordingly would fit better general expectations and the icons used.