I'm not totally sure about the specifics since despite being an engineer, I cannot distinguish electricity from witchcraft. That said, I do know that if you want to charge your phone from your dynohub you need to ideally have some sort of powerbank in the middle.

I think the problem with having a higher wattage is that you can't have a substantial amount without introducing what would generally be an undesirable amount of drag. It would certainly be useful to be able to engage / disengage a dynamo hub at a higher wattage though.