A few suggestions:
Locomotives:
Has anyone noticed the yellow item on the back cover of the 1975 Hornby catalogue? https://i.ebayimg.com/images/g/Xn0AAOxyVLNS-gxk/s-l1600.jpg
New Manor - it will be either red or blue box , but it will happen - Only 15, half of the class modelled by Mainline/Bachmann Branchline
EMU
An new EMU - They don't have to look to far for inspiration
Bullied 4 SUB https://theisleofthanetnews.com/wp-content/uploads/2018/06/4sub-jh5-615x381.jpg - YES PLEASE!
LMS 3 car (BR class 503) https://www.railwaymagazine.co.uk/wp-content/uploads/sites/24/2018/07/RM-July-PR-p11-750x360.jpg
Coaches
Maunsell 58' coach chassis - what else can you put on it? - Well: http://www.semgonline.com/coach/pics/rs_s5718s.jpg
Collett Gangwayed Brake BG https://www.rcts.org.uk/features/carriages/index.htm?id=carriages/gwr/bg
Could we see a BR Mk1 BFK and RSO?- (The blue box brigade are reviewing their Mk1's - Updated 57' Mk1 Suburbans from them possibly, including new SLO and CL?)
1951 Festival of Britain - Golden Arrow Pullmans (oblong toilet windows) https://encrypted-tbn0.gstatic.com/images?q=tbn:ANd9GcSkeKrKMSVbuj8Jq1jdj-Svp97JAy4C1ikLakf9fnlen6srjOgn
Wagons
Hornby to continue to introduce one new traditional wagon type per year. Two suggestions:
SECR Dance Hall Brake Van https://c1.staticflickr.com/6/5150/5609890176_ca4645e5da_b.jpg
BR Palbricks https://paulbartlett.zenfolio.com/palbrick
BR/SNCF MoS Dai 1/112 Cupboard Door 16T Mineral Wagon https://paulbartlett.zenfolio.com/brsncfmineral
Oh and all those deliveries that have slipped for 2018 Blue MN, Pecket H&P pack, etc