Something is clearly happening to Cortana. Bungie has explicitly referenced that she is Gravemind's captive. It seems that some of Cortana's dialogue in the Announcement Trailer is accompanied by Gravemind's voice saying the same words. However, Rampancy, as defined in the Marathon universe, is a kind of uncontrolled expansion an AI undergoes. Durandal's rampancy in the Marathon series led to megalomania. The closest comparable concept discussed in canon Halo universe material is the 7-year lifespan of so-called "smart" AIs, after which they begin to think themselves to death, a consequence and process much different from Rampancy. Rampancy is never referred to by name in any canon Halo universe materials. It is mentioned once in the I Love Bees radio drama, but not in reference to Cortana. It seems more likely to me that Cortana is being influenced by Gravemind in some way. It may be that the two will solve their respective existential problems; Cortana's lifespan will be coming to an end soon, no doubt hastened by the absorption of a huge amount of data from Forerunner and Covenant systems. On the other hand, the parasitic nature of the Flood and Gravemind clearly places them at odds with.. well, the whole rest of the universe. If Gravemind were able to fulfill what seems to be his purpose or function-- the preservation of the memories and possibly even personalities of dead individuals-- within the confines of an artificial intelligence, rather than through organic hosts, and Cortana were to be given access to an organic host for her intelligence... who knows?