The girl drop from Boss battles is random and also very low.

At this time, there won't be any new content (that I know of) till w4 comes out. Which is slated mid September.

Stamina, at this time, can not be used for anything if you have completed the story. Though it is recommended to still level since stamina spikes in W4 are still expected to exist.

The low amount of items is a concern for the devs, however, I don't think that it will be addressed before w4 comes out.