I've been hearing rumors that forgotten characters will make a return in series 11... as for Duck, Oliver, Toad and Boco, the only ones I heard that where returning. But I'm not trusting yet until there is a proof. Anyways, I can assume these characters in S11:

The Steam Team
The Fat Controller
Lady Hatt
Annie & Clarabel
Bertie
Harold
Troublesome Trucks
Whiff
Hector
Silly Billy
Salty?
Cranky?
Harvey?
Rocky? (cameos, I know)
NG Engines (Freddie?)