I began to think about how even as so many other things have been democratized, automated and made more accessible, as far as I know, billboards are still a very manual, very expensive prospect.
I can't (again, as far as I know), go online, pull out a credit card, type in my message, upload some graphics, and see my billboard in a few hours. But I should be able to.
Here's my proposal: A billboard printer.
At the top of the billboard, a roll of recyclable fabric. At the bottom, an empty reel. A motor that can roll the fabric either way.
Along the top edge, a very large ink jet printer. (Hello, HP?) Connected, of course, to a cellular data connection. And some solar powers or an electrical connection.
You pay a base price for your ad and then you pay for each hour that it appears. (Most likely new ads would be printed in the middle of the night. Or, you can buy open hours same day at rush rates.)
Certain hours would cost more depending on traffic potential. The further in advance you buy, the cheaper it is and the more likely you are to secure the timeslots you want. Likewise, it's pretty cheap if you've already got an ad printed and there's open spaces same day.
Somewhere along the way a combination of human and automated processing determines if an ad is appropriate to a particular locale. And the full-service model (design and placement) exists for those who don't want to DIY.
Every so often the rolls are swapped out and washed, or the ink is designed to fade on its own after a certain amount of time. (That would then need a camera to make sure the quality is still good.)
Anyhow, I'd love to patent this (if no one has already), but on the other hand, it would be cool enough to hear that someone liked it well enough to create a proof of concept.