That maybe seems a little complicated from a UX perspective, but I think it could be made relatively seamless. There could be a standard post form, and the service/subdomain you're using changes depending on what options you invoke. Clicking "add image," for example, switches you over to photo.foo.bar. But the option to add image only shows up on your form if you connected to the photo service. Which would be a relatively straightfoward toggle in your account settings — if foo.bar offers photo.
What's the appeal? For one thing, you'd be able to connect to all of these services from the same account, rather than setting up accounts on multiple instances running different services, or compromising on an instance that kiiinda does most of what you want.
But it would also let followers pick and choose which streams they want in their timelines. Maybe they want to read your thoughts, but don't need to see your checkins. Maybe they just want to see the links you post. Or the photos.