I gotta keep the faith.
I think there's plenty of room for the PBBG world to grow, but I also think we're going to witness the demise of many knock-off BBGs over the next couple of years. I think what we're seeing is the largest parts of the PBBGverse becoming stale. There will always be a place for text-based games, but I think as Flash pushes the bar for web games, non-flash PBBGs are going to have to boost their javascript and HTML graphics if we want to keep anyone's attention. And while Flash has it's merits, I think that the games that stick to HTML/js have more flexibility and more agility.[1]
I think the current stage we're in is the addition of AJAX to the toolkit of the PBBG developer. AJAX allows us to give our players a much faster and smoother experience. The abundance of well-tested, open-source javascript libraries makes adding effects to our games almost trivial.
The next stage I think we'll see is API-enabled BBGs. When writing games using AJAX, we take on a new mentality, the idea that it's not just dynamically generated HTML anymore. We start to see the reality - HTTP requests hit our server, our server does some processing, and it sends a response - but that response might be HTML, javascript, JSON, XML, etc. There are a lot of possibilities for adding an API to a PBBG, and I think that may be one major feature that separates PBBGs from the rest of the gaming world.[2]
I'm thinking of experimenting with some API-enabled features in my game and see where it goes. Stuff like XML-based character sheets, XML lists of items for sale in the market, etc.
[1] This statement is probably 99% my own personal bias against Flash. Flash allows tons of people to create tons of mediocre "arcade" games, but there are a few PBBGs out there that use Flash (AdventureQuest, BattleDawn) - but I remain unimpressed.
[2] It's entirely possible for non-web based online games to embrace API possibilities, but I feel like most people just don't get it. WoW has this site called the Armory,
http://wowarmory.com (NOTE: I never have and most likely never will play WoW, I just found this interesting), and if you do any searches and browse source, you'll notice that everything is XML data (it's the XSL stylesheets and javascript that make it so pretty). However, I don't see any sign of a publicly documented API anywhere. Blizzard seems to be poised to experiment with this concept, but perhaps afraid of abuse? I don't know...