[RobGR]

I think there is room for innovation in blogging platforms, and as a general rule, I would not make any feature that was not supported by a well-thought out API.

My basis for saying that there is room for innovation is that I have worked on a number of big websites, and I have never been able to use out-of-the-box blogging for any of them, the features had to be too integrated and specialized.

One way to think about the market is to keep in mind that what people mean by "blogging" may be slowly evolving.

If I were implementing a blogging platform, I would use Drupal because that's what I'm most familar with. I think extending Wordpress is also an idea you should consider. However, it be best to ignore the underlying platform, and just think about and write the API. After you have defined that, pick a platform or framework and implement it.