I am working up a Halloween Horror ghost story adventure and need to work out the ending.

The begining is the typical stop at an isolated farm, meet the family, what could be more normal and boring? Except the empty place at the table for the daughter no one will talk about.

The middle starts with a little girl at the front door in the middle of the night, dripping wet on a dry night. She walks to her room and disappears leaving a trail of wet footprints. The hired hand tells the PCs the story of the little girl drowning in the duck pond and that her parents "clean up after her" as best they can, but the players can end the haunting if they bury her bones beneeth the old apple tree.

Surreal nightmare stuff happens as the farm becomes something out of nightmare. Zombies rise from the cabbage patch, the bull in the barn turns in to a minotaur, the tree attacks the PCs, etc.

Morning is where I am not sure what to do. Options are 1) they wake up in a long abandoned and decayed farm, 2) they wake up in a farm back to normal but devestated by their nocturnal actions, 3) they wake up and everything seems normal.

I have two options for dead characters 1) they are really dead and remain so in the morning, 2) they wake up in the morning.

I favor characters who die being actually dead, partly because part of the backstory I think is that the "cleaning up" the farm family do is making the dead people the ghost kills into jerky, meat pies and Farmer Vincent's Fritters.

If I go with ending option 2 or 3 above I am considering having Farmer Vincent offer to help bring the dead/injured into town, but spin his truck across the road partway there to "take care" of the rest. Can't have anyone casting suspicion now can we? He will of course be very apologetic about this.

In any case the one mystery left for the players will be a small rag doll that is soaking wet despite the dry weather...

I originally wrote it as a fantasy solo adventure with different choices leading different endings. So the "reality" of what was happening could be different on subsequent play throughs depending on what you did.

Not quite so easy to pull this off in a multi-player table top game.

And yes, the whole things is meant to be like an 80's horror flick. So lots of cliches and call outs there.

In the eventuality a PC is killed. You could just have them make a system shock roll or equivalent save, like CON or WIS to resist the shock of experiencing death or believing they were.

If a PC dies overnight they might appear at the table in the morning, taking the daughter's open seat. The family matter-of-factly regards them as one of their own and the ex-PC doesn't deny this at all.

A clue towards this ending would be if the farm family doesn't resemble each other, maybe they're all just casualties of past parties slotted into the family roles.

I like the idea of all those who "survive" the adventure being surprise-killed in the last act. Very 80s horror (think Phatasm I or II).

If this is an ongoing thing, the next session (next halloween?), have a completely different story set in the same county. That farm? "Oh, it was bought by some big agricultural conglomerate decades ago and all the buildings razed. It's just part part of an endless expanse of fields now." Or so says the gas station clerk who sells you Nabisco Farmer Vincent Fritters. Perhaps he also looks like one of the former PCs.

Is this an ongoing campaign? Long-term? Or a one-off? Modern or medieval or?

You could have a point where PCs start having to make saves (possibly rolled by you without telling them) or fall asleep, after which point, what's described to them is a dream sequence. If you have some PCs fall asleep before others, the asleep ones could be replaced by illusions for the awake ones. While there is a mixed asleep/awake group of PCs in the same place, the asleep PCs dream about what happens in the waking world, but if they get hurt/killed, it's really happening to their illusion.

Similarly, if a PC actually dies, they could be replaced by an illusion, or ghoul with glamour, in the morning, so it seems like it was just a dream, but it wasn't. The PC's spirit could even still be dreaming along and controlling the replacement, so even the player doesn't know and thinks they're still alive.

If it's an ongoing campaign, and you want them to have a possible way to mitigate or undo a death, you can have dead PCs' spirits in a soul jar in the basement, so they can come back for it if/when they figure out what's happened.

Or it could all be dream spirits. The PCs actually camped that night, or stayed at a normal farm, but forgot that part, and had a group dream spirit visitation that seemed real and has spiritual effects and real clues, and uses the faces and location from their memories of the real farm, but then they wake up in the actual farm, it's of course in the normal state, which hopefully the PCs figure out before they do anything too bad...

This will be Modern, probably a one shot, although if the players enjoy it I could be persuaded to run a campaign.  Doing it as a one shot frees me to kill off as many as I like..

I like some of these suggestions. I'll definitely post a game report after.

Would fit in Gumshoe. There is an contest to make a one-page adventure/module for it going on still I think.

OK a few ideas:
*You could add some more complexity here by adding a second monster or demon of some kind. e.g. there could be some sort of evil spirit that lives in the pond and that drowned the girl; so her ghost hangs around but isn't actually responsible for most of what goes on.  
* the ghost just wants people to play with her, but if they do its astrally and they leave behind a comatose body, which the farmers make into pies (not realizing they're not dead and might just wake up in a day or two, or not caring). Maybe people killed by tree/minotaur/etc shenanigans wake up normally though.
*the girl is one of twins, so the PCs could be confused by their being a ghost girl who looks just like the living girl.
* Does the haunting end in the morning, or does it end when they bury the bones? Alternate endings could be for if they do vs. if they don't.
A twist might be if handling the bones they find evidence of her being murdered rather than just drowning (ropes, or fractures to the bones which might have happened before the death, maybe). Maybe she remembers being murdered if the bones are uncovered and zombies/monsters attack the farmhouse, or at least whoever was responsible.