Google has been running a preview program to let developers build add-ons for its Docs, Sheets and Forms productivity tools since 2014. That option has now been opened up to all developers.
"Starting today, any developer can publish an add-on," product manager Saurabh Gupta wrote in a post on the Google Developer blog. "To ensure users find the best tools for them, every new add-on will undergo a review for adherence to our guidelines before it’s available in the add-ons store."
For developers, that opens up potential new markets -- though given that the vast majority of Docs/Sheets/Forms users aren't paying, the best earning potential is likely to be in specific niches. For end users, it means that we may see an expansion of Google's suite apps, which could help fill the still very evident functionality gaps in the product.
It's not an unfamiliar strategy; Microsoft Office had third party add-ons well before anyone thought about browser-based apps. Arguably the biggest risk is that a feature built as an add-on then shows up as a native option in the suite.
Docs, Sheets and Forms add-ons now open to all developers [Google Developer Blog]