Add modifier that gives tools a chance to break on each swing
When a tool is at 0 durability, it has a 1% chance to break upon its next swing, increasing by 1% with every successful swing. This could be indicated by the durability bar filling with red, showing its chance of breaking (and its "chance of breaking" being shown in the tooltip.) This would be a buff to tools that swing slow, but output high damage.
(optionally) It can be a risk/reward factor, where using the tool at this point deals critical damage (#1559), but you don't know when it could break.
Edit: The critical damage thing could also be a modifier
I think this could be rather difficult to balance around.
I think it would be better to have a modifier that adds a chance to break on every swing.
I wouldn't be opposed to that at all! I think having fun modifiers like this would be super fun.