I am also a big fan of the bivy. I've had a few but found them constricting. I sleep on my side(s) and would often wake up with them twisted around me. I use a quilt and like having my pad and quilt held together in the bivy. It also adds a bit of extra warmth and protection from the dew.
I decided to make my own roomier bivy out of tyvek (the softer type, not the housewrap), double sided carpet tape, and velcro, based on ideas I saw on youtube. I've been pretty happy with it. It is roomy enough but still keeps me enclosed. It's also very lightweight and cheap. I wouldn't want to sleep through a night of rain in it, though.
One thing about the design is that the velcro sticks to itself better than to the tyvek so i took it off. I thiink I'd do something different if i need it to close up, but that hasn't been important to me lately. I've thought about putting a screen window in it to watch stars bug-free, which would be very easy to do.