If I had a Twitter account, I might have submited Anonymous as Servant Assassin, though; they might well have acquired enough of an inflated legend to qualify as an Heroic Spirit. Noble Phantasms would probably Legion: the Uncountable Collective, that caused ten new identical Servants to arise whenever one is killed (and which can merge later on to save on prana costs), and Low Orbit Ion Cannon: Wrath of the Internet United, which has a Rank proportional to how many people are using it on the target. Also, artwork is easy to come by, which'd let me compensate for shitty artistic skills.