The engine is open source so the GitHub issues is a good first point of call. There is even a label for ‘First PR request’ to get you started. Issues · playcanvas/engine · GitHub
Other developers have taken to extending the engine by adding features. eg:
Terrain editor: New release: Tiled Terrain Manager v1.20
Blend!: RELEASED: Blend! 3D Animation Blending For PlayCanvas 1.0.1
Even working out a way to develop locally: TEMPLATE: ES6 | Version Control | NPM | PlayCanvas
Unofficial Chrome Extension: FREE: PlayCanvas Editor Extension v1.4.3
The engine can also easily be monkey patched if there is a some unique to your application.
GLTF support is being looked into: https://playcanvas.com/project/512016/overview/gltf
Some developers are also integrating other libraries like p2.js for a lightweight 2D library: PlayCanvas 3D HTML5 Game Engine