An old idea I’d love to see come back would be the mashup. There was a moment where it appeared possible that all the ‘next big things’ would be built using a mix of services. Gmail mixed with Facebook chat mixed with RSS feeds all overlayed on a Yahoo Map with contextual YouTube pops. In the end all we really got were Google Maps next to physical addresses on websites, social share buttons, and Facebook Logins. (I know there are more but wide adoption is scant).
It would be killer to see a collection of micro-mashable services emerge to expose data, manipulation, visualization, sharing and interactivity (and probably billing).
It should, for example, take a day to build a page that lets you analyze real estate deals based on proximity to McDonalds locations, crime events, R v D political donations, and mentions (or lack of mentions) of Kanye West by local Social Media users.
By the way, I’m not saying this is a good idea, it is probably a bad idea. I’m just saying ideas like this should be almost as easy to hack together as a formatted blog post.