(Insipired by forthcoming change of commute from one tube, to one tube, two overground trains and a bus)
Personalised travel news for your route to work. Looking at the travel feeds, it seems different overground train routes have different ID codes in the XML, so would just be a case of picking the right bits out of the XML.
Obviously could be modular so your personal details could be made up of overground rail, tube, roads or whatever. If you wanted to be really fancy, you could add in Jamcams in somehow.
Useful and not particularly difficult - just need to work out the different routes from the XML(!) as a basic prototype, and if really keen, scope for a config system and so on. Will build basic prototype if you're /really/ lucky :)