We now have the '+' Icon, but it would be nice if we have the '-' icon too.
Agreed! The plus icon can be associated with the "X" (Cancel) when you want to add or delete something. However, it can also be associated with a numeric text box where you want to add one to the number or subtract one from it. The minus icon would be nice to have in that case.
This would also be nice to have when a form has expandable/collapsible sections. Essentially when a section is collapsed the icon control would be set to "Add" icon to signify that it can be expanded and when a section is expanded the icon control would be set to the "Minus" icon to signify it can be collapsed.