I honestly think that any of the following would be great additions to the Thomas line. Especially if Mattel would just lift the CGI only rule as and expand the options for characters instead of multiple repaints.
Engines:
Flying Scotsman
Murdoch
Shane
Nia
Ashima
Stepney
Derek
Boco
Rolling stock:
Flying Scotsman water tender (if sold separately, this would definitely sell even within non Thomas modelers)
Troublesome truck #7 (standard gauge slate wagon as seen in Thomas train to the mainline in journey beyond sodor)
Rickety
Fred pelhay
Silver tanker
Old coaches
Troublesome truck #8 (teal salt wagon also seen in Thomas' train in JBS)
Bradford
S.c.Ruffey (rebuilt version)
Narrow gauge engines:
Duke
Smudger
Freddie
Mighty Mac
Narrow gauge rolling stock:
Green open coaches
Brown brake van
Flatcar
Engines:
Flying Scotsman
Murdoch
Shane
Nia
Ashima
Stepney
Derek
Boco
Rolling stock:
Flying Scotsman water tender (if sold separately, this would definitely sell even within non Thomas modelers)
Troublesome truck #7 (standard gauge slate wagon as seen in Thomas train to the mainline in journey beyond sodor)
Rickety
Fred pelhay
Silver tanker
Old coaches
Troublesome truck #8 (teal salt wagon also seen in Thomas' train in JBS)
Bradford
S.c.Ruffey (rebuilt version)
Narrow gauge engines:
Duke
Smudger
Freddie
Mighty Mac
Narrow gauge rolling stock:
Green open coaches
Brown brake van
Flatcar