In 1/72 it's real easy:
Brooklands/Edgely Optica
Piaggio P.180 Avanti
Handley Page/BAe Jetstream series
DeHavilland Canada DHC-7 "Dash 7" and DHC-8 "Dash 8"
I know the Optica and Avanti have been out there in resin and I don't expect the Optica to ever be available in any other medium despite recent attempts to revive production of the type. I would have though that Italeri or Revell Germany would have furnished us with an injection molded Avanti by now though.
Airfix hada couple of militarized Jetsteam kits in 1/72 years ago so it would be nice to see them revisit the type with today's tooling technology.
I once tried to build Hobbycraft's 1/72 Dash 8, but I'll be happy if I never see that kit again.
I grew up watching Dash 7, Dash 8 and Jetstream 31 types coming and going from my hometown's municipal airport and would lave to have good representations of them in 1/72
In 1/144:
A really good long pod 737-200 would be great. It's another type I saw a lot of growing up. if it could come with a Canadian specific decal sheet with Pacific Western, CP Air and Canadian Airlines marking options, so much the better. I still maintain the white over polished metal scheme with two tone blue trim of Pacific Western was one of the most attractive schemes ever put to an airliner IMHO.
Back when they weren't afraid of colour on airliners!