Most of the time, I expect to get
Kit - A small handful of cas or bb (not both) leftovers from other packs
SP - Some cas and bb, a small amount of new interactions and gameplay, maybe a related aspiration and/or career (e.g., Paranormal Investigator) or a fairly simple new system/feature (e.g. butlers, tiny homes)
GP - More cas and bb than an SP, more new interactions and gameplay, maybe a couple of aspirations and/or careers, a new trait or two, a substantial new system of some kind (e.g., character traits) that may or may not be pack-specific , maybe a new world and/or new lot type
EP - Max amount of cas and bb, substantially more new interactions and gameplay, more than a couple of new aspirations/careers, a few new traits, a couple of substantial new systems that have cross-pack potential (e.g., fame), maybe a new lot type, definitely a new world (except for Seasons)
So far, Horse Ranch feels like a GP to me. Similar to the occult GPs, horses are a completely new sim type with traits and skills and their own horsey events and such, so maybe all the horse-specific stuff is taking the place of a new system that would apply across packs, which I'd expect to see in an EP. But, this pack is more limited than the occult GPs because at least human sim occults can also do/be whatever non-occult human sims can do, so they have more cross-pack potential. I think that's the thing I don't like about it being an EP. If you decide not to include a horse in your household, it seems like there's very little else from the pack you can integrate into your gameplay besides the cas and bb (and nectar-making, which seems very similar to juice fizzing to me). It's a walled garden, like what I imagine Batuu to be.
Well, maybe the livestream will reveal more that I'm not aware of yet...
