I'll give you some details on who the game will be:
Your caretaker is Fluttershy.
The pun maker [ sans ] will be Pinkie Pie.
The spaghetti lover will be [ Papyrus ] will be Twilight Sparkle.
Scootalo will be Monster Kid.
Rainbow Dash will be Undyne.
Rarity will be Mettaton,MettatonEX.
AppleJack will probably Asgore [ if i find another pony to be Asgore i'll make sure i tell you guys ].
More Details:
The volcano looking thing that's in hotland [ sorry don't know the name ] will be spitfire.
You can also name your pony as well.
The Echo flower will be Derpy. [ your probably thinking why Derpy but i really just thought Derpy to repeat things. Sorry if it's a bad choice. ]
Bulk Biceps will be Aroan [ Sorry if i spelled one or both names wrongs. ]
More little details: Starlight Glimmer will be Muffet.
FlowerWishes will be Whimsun.
Lyra will be Shyren.
BonBon will be OnionSan.
More Details Coming Later.
Even More Details:
I will work on the music for the game so it can sound like the actual music from UnderTale.
I might have voice acting.
I will have the character dialogue on the top or bottom of the screen.
Its not really a Holloween game it actually coming out on Christmas because ween i finish the game on Halloween then i probably will change some stuff and then it might be ready for Christmas.