Sunspots are cooler areas of a star's surface that appear near black when viewed through certain filters. They are usually a byproduct of solar activity (magnetism) which also causes things like CMEs and solar flares.
That said they are already in the game. You don't really "see" them for a couple of reasons. The first is actual physical appearance. Sunspots are not black they are just darker areas of a star. There is no way that anything that is undergoing nuclear fusion will not emit light and thus be black so it is just a darker color than the surrounding area. The second reason is distance. In ED we have the ability to travel almost near the surface of a star. At that distance sunspots (read colder areas of a star) will appear as a slightly discolored area.
Keeping all this in mind I'd love to see in the exploration side of the game (potentially in combat) scanners that can actually alter the way your view looks. If you ever played one of the Aliens vs Predator games where you play the predator you know exactly what I'm talking about. In Elite Dangerous ideally if you have an advanced scanner, as you scan an object (star, planet, ship, whatever) you cycle through different view to give you something of a wide range of spectrography. To keep it simple it would have to go through presets like heat, water signature, ammonia or whatever but in one of those views you'd be able to scan a star and get your dark sunspots.
So that's the reason eh?