Like Thanos and Surtur (if Surtur was ever to appear), Dormammu would likely be introduced in to the Marvel Cinematic Universe as a behind-the-scenes mastermind initially - and he would therefore need a minion to do his dirty work for him. Loki fits the bill in this case because Dormammu is practically a god of magic and Thor's adopted brother could stand to learn a lot from him and become more powerful. If Dormammu was to emerge first as a foe of Doctor Strange in the Marvel Cinematic Universe, he might want to send an apprentice to take on the Earth-based sorcerer before confronting him himself - and Loki, having already shown his ambition in invading Earth with the Chitauri, may have impressed Dormammu enough for him to call upon him. With the incentive of giving Loki a prize - such as the Book of the Vishanti or an item of equal power - Loki would no doubt be keen to do business.