A lot of it would depend on if they remain true and consistent to the other movies. But assuming they do, I think it's highly likely Shorty will return. Bron did adopt Shorty after all, so his absence would be very conspicuous from this movie. I can easily see Shorty letting the gang know that Bron is in trouble or something, and accompanying them to find him. Plus, Shorty's VA already changed once, so it's not hard to imagine them changing it again. And if Shorty does not appear, they're going to have to explain why he's not there, or else it would be a pretty big plothole and a sign of broader problems with consistency.
As for Chomper and Ruby, I could see it going either way. They (especially Chomper) are pretty popular and were main characters in the TV shows, but since many of the viewers might not be familiar with the TV show (or even saw 2 and 5), I could easily see them being cut in order to put a greater focus on the original Gang of 5. On the other hand, new characters have been introduced in all sequels thus far-- perhaps Chomper and Ruby can be the "new" characters of this film, especially since Chomper hasn't appeared in the past eight movies and Ruby hasn't appeared at all. If they do appear, it would be nice to hear their backstory a bit more. Perhaps they could meet on the journey if the movie would take place before the TV series.
If (and probably only if) Chomper and Ruby appear, I think there's a chance Redclaw would appear as well, especially if they're angling for a more suspenseful movie. Since he has a reputation of being a terrible sharptooth, his presence could heighten whatever danger Bron would be in. Of course, if Redclaw appears, it would probably mean Screech and Thud would show up too.
Potentially I could see some of the family members not showing up. If they're going for a darker tone, Grandpa could perhaps have died (because his VA did) and maybe that has something to do with why Bron is appearing. Still I think this is unlikely, they'll probably not want to go that dark, and I think they'll just recast Grandpa just like they did Grandma. Similarly, I have a hard time imagining Topsy (whose voice actor also died) go, especially since he's such a vocal member of the herd. Plus, Topsy's too young and I can't see them killing off multiple adult figures. Also if the movie is largely set out of the Great Valley, I could see many of the parents (especially Mama Swimmer or Mama Flyer) show up, but not have speaking roles. I could especially see this happening if they have lots of recurring characters, like movie 4-- many parents did not have speaking roles there, and there were 3 guest characters, not to mention 2 speaking villains.
I think there's a good chance for a new character or two to show up, as there has been a new character in every sequel. On the other hand, the use of recurring character(s) in this movie make me think this is less likely. Perhaps they might go for an Elsie, giving a new character a very small role. After all, Elsie is from the only movie featuring a previous guest character. There's perhaps a small chance I could see for Ali or Pterano to return since they're pretty popular among fans, but adding them would greatly complicate the movie and could stuff too many plots within one film. Plus, they haven't returned for many films since, and with a plot focused on Bron, I don't see them returning now.
Here are my estimated probabilities of characters appearing:
Littlefoot (100%)
Cera (100%)
Ducky (100%)
Petrie (100%)
Spike (100%)
Bron (100%)
Grandma Longneck (95% appearing, 90% speaking)--I can't really see her not appearing.
Topsy (90% appearing, 85% speaking)--VA death, but he's probably too large of a character to cut.
Mama Swimmer (90%)--As long as they spend some time in the Great Valley, she'll be there
Mama Flyer (90%)--See Mama Swimmer
Tria (90% appearing, 85% speaking)--If they have Topsy not speaking due to VA death, she might not speak either.
Tricia (85%)-- I imagine she'd be there, at least briefly
Grandpa Longneck (80% appearing, 75% speaking)--I imagine they'll recast Grandpa, but I'm not entirely sure.
Shorty (80%)--It would be a pretty big plothole if he doesn't show up, but you never know...
New Character(s) (75%)--There's been a new one for every movie, but if this movie is heavy on recurring characters, I could see them forgoing one.
Chomper (35%)--Fan favorite, and part of the gang of seven, but they might choose to stick with the gang of five.
Ruby (30%)--Similar to Chomper, a little less likely since she hasn't appeared in any movie at all yet.
Mr. Clubtail (30% appearing, 25% speaking)--He's there quite a bit, if they spend a lot of time in the Great Valley, I'd boost his chances.
Redclaw (10%)--Decent chance if Chomper and Ruby appear, but only then. They'd probably more likely use a generic sharptooth.
Screech and Thud (9%)--Will probably only appear with Redclaw.
Mr. Thicknose (5%)--Larger chance than most one-movie characters because of fairly big role in TV series
Ali (2%)--Fan favorite
Pterano (2%)--Fan favorite
Others (1%)
Yellow Bellies (<1%)--I can't see them bringing them back again!
Edit: Raised Chomper and Ruby's probabilities from 25 and 20 to 35 and 30
