Here is a feasible idea of how an initial version of one might work. There would probably be a lot of iterations. (Making this all up on the spot, I've done zero research
The simplest use case is information transmittal and retrieval. This is especially useful if accompanied with permanent monitoring and recording, maybe perhaps in something like an append-only log, so aspects of brain state can be tracked, stored, simulated, and replayed. For example, users could possibly be trained so that their "inner monologue" was always being "heard" and interpreted by the interface, so it could be immediately and permanently stored remotely.
This could/would probably just go over the internet, protected with a TLS-like network protocol. This could allow some degree of perfect memory recall, and replace/augment all productivity and note-taking applications. Some kind of "mental command" prefix before a thought would search the memory database and present a result in milliseconds or less.
Internet resources could be retrieved with the same speed and ease-of-use as personal memories. Users could "IM" via thoughts with latency close to the speed of light (plus however long one takes to think a thought), and possibly transmit other crude symbols ("mental emojis", custom 32x32 images interpreted from mental imagery, etc.)
Already with these features you'd get some interesting things. Then use some kind of bizarre new very high-level programming languages to improve the ability for critical thinking, maybe even artistic intuition, and then you'd maybe begin seeing some self-singularity type of developments. The improvements might peak quickly, but they'll probably look back on us today like pitiful cavemen