For loose summer pants, MUJI has a pair of hemp tapered pants that aren't too bad.
They are fairly loose, however they run short if you wear them at your natural waist.
Not sure how the durability will be, since they are lightweight, but I've got a few pairs of their hemp shorts that have lasted over 5 years (but I don't use a clothes dryer), and they are really nice for the relatively hot and humid summers we have here in Japan.
Another caveat: They are produced in Cambodia, so I'm not too sure about the labor practices/conditions there.