This is just something that I was thinking about but I'm not sure if it would be doable or even liked by the players. Some games I have played have voluntary ads you can watch to give you some reward.
There are a few incentives they could give for it. Kobans, temp drop rate increase, an affection item are a few. It would be a way for players to be able to support them and for them to make a little extra money.

I'd like to hear what other people think. Would you watch an ad if it got you something a little useful? Do you think it would be able to work?