I will try to compile a character list from the posts I have seen on Social Media so far.
Treat Trail characters start when the trail officially opens or shortly thereafter, that matches my experience from last year. Meet and Greets usually don't start before 6.30, some have been delayed to 7 in the past, the main one being Carthay Circle, probably because that is the last spot where the non-Partyholders will be in.
Treat Trails:
Doofenshmirtz - Disney Junior Building
Oogie Boogie - Disney Animation Building
Hades - Backlot Stage
Ernesto de la Cruz - Hyperion queue area
Yokai - Wharf/San Fransokyo
Madam Mim - that little hideout behind Carthay where they shove 2 Food Booths for Food and Wine in
Judge Doom - behind Soaring
Agatha Harkness - Sanctum on Avengers Campus
Sid - Pixar Pier Bandshell
There's non character lines in the World of Color area and in Carsland, and some stops on Buena Vista Street.
Meetable characters:
Carthay Circle: Pain & Panic, Hook, Hans, Sheriff of Nottingham, possibly Prince John, Old Hag Friends were told there might be surprises. Last year Hercules was out, and later in the night Megara joined him who no one had reported seeing before. I had a pretty spectacular nervous breakdown, she was my character unicorn at that point.
Roaming near Disney junior: Evil Hag from Snow White, I have also seen one lone picture of Pinocchio's Blue Fairy.
Avengers Campus, some roaming, but mainly the picture spot: Scarlet Witch, Thor, Loki (TVA outfit), Black Panther, Spiderman, Star Lord, Black Widow. One lone picture of Mister Knight, he was in his usual picture spot - when you walk out of Campus in the Grizzly River area there is a little alcove like place to the left, end of Spiderman show viewing area. Star Lord is possibly at the Dance Off area.
Next to Monsters Inc. ride, that line will be visible pretty fast: Minnie, Daisy and Clarabelle as the Sandersons
Cars Land: the usual cars in Halloween costumes, I think they wear the same throughout the day
Water Wheel near Grizzly River Rapids: Bruno
World of Color viewing area, down by the water: Kingdom Hearts Mickey, Donald, Goofy. They alternate, but if you are lucky you get a duo pic. I have yet to see all three together.
On Pixar Pier near the Inside Out ride: possibly roaming Toy Story characters, saw 2 pictures of Bo Peep and one of Woody posted with an Oogie Boogie Bash tag.
Pixar Pier (where I can't find a closer description of the spot and that one photo posted looks like the Villains Night event): Lotso
Gazebo on Pixar Pier (near the Carousel): Ember and Wade from Elemental, only seen solo pictures so far, so they may be alternating.
Near Incredicoaster: Frozone and Edna Mode
I think that is it.
If you want to see the Sandersons I personally would get in line no later than 5.30 PM. Well they are probably not officially letting the line form, but there will be tons of people lingering in some sort of "unofficial line". That line never dies down, in fact there have always been reports it was cut off early throughout the last 2 years.
Next popular would be Kingdom Hearts and Bruno, but those can be timed right with parade times.
The Carthay Circle spot gets pretty crowded towards the end of the night.