One of the UK mags I borrow from the Library does an annual questionaire on what readers want - the #1 OO locos (steam and Diseasal) seem to appear about a year later. Only what I would term a very small number (300 odd) of people respond to the questionaire.
I don't think there are many common components anymore, other than couplers, motors and gears. Way back Tri-ang had the Jinty 0-6-0 chassis, the Princess 4-6-2 and the Bo motor-bogie. It was amazing what a range of locos they could produce from those. Long may they be forgotten!
Greg.P.