2. Another Mental Duel
The possibility of having a Time Lord mind to play about with would probably fill the Intelligence with unbound glee. And the battle for control between it and the Twelfth Doctor could make for a fascinating introductory story arc in the process of trying to find Gallifrey and return once more. Imagine the shock and outright horror Clara would feel if the new Doctor's first post-regeneration words were spoken not in his own voice but that of the Intelligence, who would of course be bathing in his own arrogance and calculating brilliance. Making it all the more terrifying for all of us watching at home too. The Eleventh incarnation's hubris in showing it that London Underground map, the site of a future (from its perspective) defeat rendered irrelevant to it for at least a while. There'd be no need to even tell Clara the plan though she might be an unwilling part of it and she could really come into her own appealing to the Doctor to fight back. Of course it wouldn't be one way traffic as this isn't exactly a weak mind we're talking about, but watching the duality unfold could be quite special as well as a real challenge for Peter Capaldi. Plus there's the chance that with control of the TARDIS within its grasp the Intelligence couldn't resist the opportunity to go back and reverse Twelve's past victories over it with a few other choice monumental meddlings in the process. Possessing him to set course for the Det-Sen Monastery or the aforementioned Underground could have terrible consequences for his Second self and in turn all the others. The threat of death in all corners of his own timeline made terribly real by his own hand is just one of many possible consequences of such actions. With a run of at least thirteen episodes confirmed for Series Eight next year there's plenty of time for a few detours from the planned return home and no limit as to how the Doctor could be manipulated. Which makes things all the more fascinating whether or not you've seen 'The Abominable Snowmen' or 'The Web Of Fear' yet. Maybe after he finds Gallifrey he really does become 'the killer of his own kind' with a little persuasion..the ultimate victory for the Intelligence and a chance to radically alter the Doctor's timeline in the worst way possible from Twelve's perspective. But an interesting one for us watching at home we're sure you'll agree.