Add web hooks
Many integrations would be much easier to accomplish if HubPlanner supported the concept of web hooks. In essence what this means is whenever you change the data in HubPlanner, you could have HubPlanner automatically call a certain (user-specified) URL with a data blob (probably JSON) describing that change.
The current API model is great for reading and writing to HubPlanner, but it does not allow for very efficient integrations from HubPlanner to other things. But web hooks make it much easier to set up automatic update mechanisms, such as emailing users when they get assigned to a project, or posting a message in a Slack channel.
As an example of this, look to what GitHub does. They have an API for inbound integrations and Webhooks for outbound integrations.