I think, it will have to be tangible, else it won't really be a replacement for DVDs.
in reply to The next successful successor to DVD as a medium for video will be:
I imagine something USB sticks like, people will be able to load/unload, movies to their DVD players huge multi-gigabytes memory.
This will mean, I can load like 10 movies to my DVD player and switch between them using the remote controller, without having to make several trips to the DVD player.
This of courses, make it easier for people to share movies, so I don't think it will ever happen, but from a user perspective, I would vote for it.