GarageBand: now that you can send music to iTunes, why not have a feature where you can buy the drum + bass tracks (possibly rhythm track as well) of a song off of the music store and import that into Garage Band. It would make it a lot easier for guitar freaks like me to pick up a new song, and 99 cents would be dirt cheap. The marginal cost to the producer should also be near nil for most current bands given the digital mastering process. If they wanted to be really special, they could even distribute/sell amp and effects settings to make your guitar sound like the actual recording (Line 6 does something similar with their effects pedals).

Feed aggregator: I'm probably going to be adding a feature to my feed reader that lets you subscribe to a comments thread via a bookmarklet. If you make a post on another MT blog, it's kinda annoying that you have to keep going back to the entry to see if the discussion has been updated. This would allow you to keep up with the discussion in real-time.