What about payments (charging a small fee for transactions) on their platform? The number of people already making business transactions on WhatsApp platform is huge, specially in Brazil [1]. A big bank in Brazil has already tried to grab this market by creating a keyboard with wire transfer integration [2].
This model is very similar to what WeChat has done in China.
[1] https://www.zdnet.com/article/whatsapp-is-the-main-digital-c... [2] https://medium.com/@leandroscosta/bank-ita%C3%BA-launch-a-ke...