Forteana and social commentary.
Well creepy totem pole. Hetocumtek apparently. (Made up, judging by the first five Google hits all being SJA related.)
An effectively (or just apparently) inert threat, a cursed object something exuding a malign influence without doing anything. There are a fair number of these around the Whoniverse, several with a tendency to take people over to further their (possibly long out of date) agendas and others prone to coming to life and attacking unsuspecting museum staff.
A curse in something as simple as a name - apart from one case where the script ensures Dr. Madigan is properly introduced it naturally targets friends and family. The ultimate end to drive him out and destroy him, taking his power?
Having everyone turn on a PC... obviously it results in the party splitting. Out-of-character behaviour caused by external influence is often unwelcome among players - usually because the GM dropped it on them unannounced. "You're possessed, attack the others" rarely works. Conversely, asking ahead "want to be possessed and attack the others?" appeals to players rather more, letting them veto it if they're unwilling.
A monster targeting the most vulnerable in society?
Being homeless is bad enough without adding monsters. A game centred on homeless people (who see things everyone else overlooks) could work, although it might not be fun. It would be rather less direct in a period like the Great Depression rather than right now. Neil Gaiman's Neverwhere started like this, but moved away to a more metaphorical take on the idea to avoid making homelessness look exciting. Not a problem here, it being a real social commentary downer episode without quite becoming painfully obvious.