My client today wanted to know the various solutions of displaying external data in CRM without saving it in CRM. While I was reading about various options, I saw the upcoming new feature in Dynamics 365: Virtual Entities. It might be worth to wait for this new feature than spending time on developing Custom HTML pages/ iframes.
More to read:
https://community.dynamics.com/crm/b/exploringdynamics365/archive/2017/09/11/virtual-entities-in-dynamics-365