Decoding hand writing accurately is an approximate thing even when it's spoon fed to the machine via a digitizer, to say nothing of interpreting the movements from neural activity. What could be accomplished if we abandoned the familiarity of writing and existing glyphs and established a novel set of learnable 'brain semaphores' (e.g. visualize this symbol, play this sound on your mental speakers) that were easy enough for the user to practice and master with feedback, and blazing unambiguous markers to the probes and recognition algorithm? It's not hard to imagine that with deliberate practice and immediate feedback, the brain can develop connections for the sole purpose of the interface that no longer even piggyback on imagining motor/sensory events. Sending a keystroke would be no different than lifting a finger.
I'm sure this isn't a novel idea, but my armchair observer prediction is that the field will start to move in the direction of purposefully designed mind-sign schemes and away from mimicry of physical movement as the technology becomes ubiquitous and less invasive.