[sssparkkk]

Have a look at what we're doing with our startup: https://platform.happening.im - it's exactly what you're describing: a (group) messenger that allows third party developers to create their own HTML5-based apps that can have complex functionality and/or interfaces.

[nostrademons]

The top feature of any software platform is "can reach hundreds of millions of users". Solve that problem first and then maybe you'll be able to entice developers to build on your platform.

[joepjp]

Actually, I recall this platform from being quite popular in the Netherlands for a while. It received quite some media attention and a lot of people I know used it. It's not hundreds of millions, but it definitely had some reach!