Though he does not specify what these additions will be, Kimishima states:

People are guessing this could refer to anything from varied game controllers to extra battery packs that would lengthen the rumored 3-hour lifespan of the mobile form of the console.

Possible speculation includes:

Swapping out the JoyCon controllers for something bigger or in a different controller style – like those of an Xbox controller for on-the-go gaming (which seems possible considering how many different partners will be involved with the console) Speaker attachments to the mobile console VR equipment

Although the accessories may allow the console to be playable in different situations or even without a TV, Kimishima says:

What accessories would you like to see available for the Nintendo Switch? Let me know down in the comments!