For some future version of IPB (or as an addon product), I would like to see IPB support an OAuth2 API (like many other sites do) that exports a good selection of services for my forum (like sharing topics, posting comments, etc.).

There are PHP SDKs out there that allow you to implement an OAuth2 server.

The reason I thought about this is that I'm thinking of implementing an app that could use access to my forum's data for a member and it would make it very easy to make use of my forum as a Web Service rather than hooking in directly to IPB PHP code.

IPBWI does most of this.

You should look out for IPSConnect in 3.4

You should look out for IPSConnect in 3.4

IPSConnect is just for authentication - it's completely useless for what he's describing, which is to use services to access and update content as well. OAuth isn't just for authentication.

I was wondering whether IPSConnect allowed transmission of "meta"/extra data too, such as reputation. Kinda like how stackoverflow/stackexchange work (although they don't share reputation across sites for anything but showing credibility of the user on their other sister sites.)

Another idea for such meta data would be sharing avatars. Although you can argue for people to use gravatars...

My $0.02.

Recently i needed an Oauth2 server for IPB and i wrote the application myself. It's .... a really small one app, no CP, no options, but Oauth (login with my IPB account) works fine and i thinks it's a base for doing more.

Oauth2 Server for IPB