So instead of selling a game that has both the campaign and MP it would be separate. For the campaign part you can buy a physical copy of it like you can normally do with games. You can also buy it online as well. For the MP that would be bought separately. That would be handled in the same way you buy online. You can either buy it online or by those cards you can find in stores. When you buy the campaign either way you get a one week trial for the MP. For the campaign there can simply be demos.

To me this would be a good idea simply because not everybody likes the campaign to a game or there's people that buy a game just for the campaign. So if you're buying a game like Halo 5 why should you pay full price for the game if you're only getting it for the story?

Sounds like a logistical nightmare.

This, and in addition, that cuts into profit a lot. The average AAA game costs $40 million to make, and you want to effectively double the copies in circulation while cutting the price in half? That would be a blunder in terms of business decisions.