An easy way to open up a platform to 3P integrations: an app store.
The app store is the only point of distribution.
Items listed in the app store go through some level of review by the platform owner.
This helps significantly reduce risk in the platform–you can assume some baseline level of good behavior from apps.
You can also cap the downside; if an app is badly behaved it can be removed from the system before doing too much damage.
Contrast that with the web platform, which must assume that all web content is actively malicious.
However, this obvious way to start locks the platform into a path that has a much lower ceiling.
The classic logarithmic-value / exponential-cost curve.
The problem gets especially bad if the platform starts off only approving a small number of featured apps.
This sets users' expectations for how trusted the apps are, which then becomes a bar that is dangerous to lower in the future.
In addition, new features will be added to the platform that assume a given level of trust in the integrations, making it harder to lower later.
The app store model puts the platform owner in the position of gatekeeper.