Here are my predictions for HO and N. 

HO Locomotives

Nia- She would complete the Steam Team, and is ever relevant in the franchise.  Would love to see it

Philip- Philip would add some variety to the line.  He looks like nothing else, and is fairly small.

Wildcard: Porter- With Salty reentering the line, it would be amazing to have his partner.

Winning the lottery: Murdoch- Not a character who is particularly popular, however with Beau setting precedent for reused toolings, he could be produced with the older Bachmann 9F tooling and lack moving eyes. I think he would certainly be a surprise 😂

Rolling Stock
I am skeptical of a rolling stock announcement, but if I had to guess...

Dogfish Hoppers- As seen in the CGI series

HO Narrow Gauge

Duncan- I think they could re-announce him with either a more prototypical look, or with plastic construction

Luke- My other guess.  He could have crossover with his prototype.

N Scale

Edward- Would complete the original Steam Team
Rosie- Can be made in two liveries

Wildcard: Rebecca- would look amazing in N Scale

Extra Wild Wildcard: Flying Scotsman- he could again reuse Gordon's chassis, and maybe in developing him for HO, some of that could make designing him in N scale easier.

For rolling stock I think they could announce Red Express coaches


Accessories

I think anything is possible, but my personal wishlist would be

Carly the Crane

Flynn (as a road vehicle, though maybe he could be made as a locomotive)

Ace

Jack

Or Kevin.