Currently, the bookmark fringe icon is a circle. However, Emacs already 
uses a circle to represent breakpoints (as do a lot of other IDEs). 
These are usually a different color, but I think it would be nice if the 
bookmark fringe icon were a different shape too. This would help 
colorblind users, since (depending on their Emacs theme and what kind of 
colorblindness they have), it might be hard to distinguish the bookmark 
icon from the breakpoint icon.

It would help make the purpose of the indicator more obvious to users 
who don't directly use bookmarks. Some packages (including the built-in 
org-capture package) set bookmarks automatically, and a user might not 
realize that the dot indicates a bookmark, as opposed to some other thing.

Attached are some screenshots showing before/after, plus a patch for 
this. I converted the string definition of the bitmap to a vector of 
(binary) numbers, since then a reader can see the shape of the icon if 
they look carefully.