One of the risks of working with a licensed RPG or setting is the idea that the source material is the final measure of things. In other words, your stories, etc. need to match with what was put down before. You can't put the Shire next to Mordor or give dwarves bazookas and still call it Middle Earth.
Lovecraft is interesting in this regard because despite all the stories, entities, and monsters the whole idea is that people don't know these things' true natures. It gives you a fun springboard to start from.
While I am working with several ideas based on actual "official" creatures for the campaign, I do think I will be homebrewing a few things as well. Another issue with licensed/derivative games is that the players may be quite knowledgeable about "the lore." While good players will avoid metagaming, an obvious tell as to what their dealing with might wreck the suspense.
[I'm deliberately avoiding spoilers here, as a potential player or two might stumble along these posts.]