I would love to be able to sign up for just HBO GO. I am more of a DVR/ondemand/streaming consumer so this would be a better fit than having 10 cross repeating HBO channels. I pretty much stopped watching the actual premium channels because they never air what I want to watch when I want to watch it.
You have to wonder however if they are going to go the Netflix/Hulu or the Amazon Video/iTunes model for the standalone client. I am sure there plenty of people willing to spend $7-8 on a all-you-can-consume streaming service as they already do with netflix and hulu. I also think a la carte service could work that lets you buy just the series you care about.