Going way back, the Brendan Frasier Mummy movies were far more action/adventure than horror, but they distilled down to a a pretty fun house. I think you're right, not having to be a straight-up "book report" can give the designers a bit more creative freedom. This ties into why multiple IPs in a single Blumhouse house seems to work well, they only have to hit the highlights.
Also The Mummy Returns (or was it Mummy 3?) probably exemplifies a third IP category, foisted on the event by Corporate Overlords. Sometimes good (Dead Silence), sometimes bad (Dracula Untold).