For an innovative take on a traditional unstructured bivy the Backcountry is an interesting option. Spacious, easy to access and good venting make this lightweight bivy a great all-rounder.
Sierra Designs says:
Updated With New Color & Styling. Still designed to fit our revolutionary Backcountry Bed and a 2.5” inflatable pad on the inside of the bivy, so you can sleep as comfortably on a rock ledge, as you do at home.
Sierra Design's Backcountry Bivy uses an interesting design to increase space and comfort. The simple, unstructured design minimises weight while the use of a thicker floor fabric ensures durability is not sacrificed. There is a good amount of internal space which allows you to move around easily enough, even with a thicker mat and bigger sleeping bag.
The bag uses two different U-shaped openings, one for access and the other for ventilation. This smaller vent opening has a mesh cover to provide protection from bugs. The Backcountry does well for dealing with moisture/condensation with fairly breathable fabric combined with the vent. There are still plenty of occasions where it does build up but this is part of using a bivy.