Carboys are antiquated, IMO. If you're trying to move away from glass (a good idea for safety reasons), there's no reason to remain attached to the shape of a blown glass carboy.
I'd much rather have a 7.5 gallon Better Bucket made of PET so I can have plenty of head space, and be able to watch the cool fermentation process. It would also be far easier to dry hop or do fruit additions in the secondary. It would be much easier to clean, as well (just pop off the lid). I'd also like it to be usable as a bottling bucket--installing a spigot would be much easier and cheaper in a bucket than a carboy.