Another idea I haven’t pursued: something that monitors the movement of local transit (BART trains, and buses) so I can check their ETA’s and locations from a phone browser. There are existing apps and sites for this but they unfortunately suck. They always answer “when will the next bus/train arrive at stop XYZ” which is almost never what I want to know.
What I want is usually either “I’m on the #123 bus RIGHT NOW and want to get off at XYZ, how many more stops do I wait?”, or else “my friend is on BART and just passed station X. When will she get to station Y where I’m supposed to meet her?”. Anyone implementing anything like that, please take it into account. The transit agencies usually have API’s to get the relevant data.