The concept is great but there are a lot of challenges to solve. You need to create custom firmware for each router model which will take a lot of time. There are too many vendor in the market.
I believe they will not touch router firmware but build upper level so that heterogeneous systems may engage in communication.
Well, that's not how router work. I was trying to do the same thing and i know how hard it is to build a system like that. Most router in the market come with fixed firmware, there is no easy way to program this kind of router. That mean most people can not turn their router to an captive portal (to serve ads) in anyway.
The second approach is to build a custom router but this make the project more complexity. There is also existing product in the market, google "captive portal" for more information.
In my opinion, this project is not gonna work or at least can not reach that billions market size.
-->