How about something that could be sold with an optional accessory kit?
Deagostini have already done this with the Spitfire, so how about something from another area.
The one that comes easiest to mind for me would be the U.S.S. Enterprise NCC-1701-D from "Star Trek: The Next Generation". Base kit, 100 weeks to build the iconic spacecraft, with an option to extend, only open if you have already completed the 100 issue run, to get the parts for the 'Future' enterprise from the final episode "All Good Things".
Or a partwork set. "Du(a)(e)ls". An iconic car, bike, warship, or whatever, and its main rival [Duels], or close ally [Duals].
A ship under Drake's command, and an equal class ship from the Spanish Armada, for example. Or both Spitfire and Hurricane.
Obviously each model in this sort of set would be smaller than the sailing vessels and cars, etc. currently produced, and there would be a risk of subscribers only picking up the issues to build the single model they want, but there could be an equal number waiting for the next model in the set to join in.
What about releasing two sets a year? Launch one set, then as that set reaches the mid-way point where it is going to change from one model to the second, start a second set. Ideally from a different subject, so, half way through the Drake/Spanish Armada set start up on Tiger 1 and T34 WWII tanks, or Spitfire and Hurricane. The list of combinations must be very long - Spitfire and Hurricane, or Spitfire and ME109? Mk1 Ford Escort and Mk 1 Ford Focus rally cars,
Current Builds
Eaglemoss: Ecto-1, BTTF Delorean [Installing Mods]
Hachette: T800 Endoskeleton
Agora Models Shelby Cobra 427 [Plate 031]
BanDai 1:5000 Imperial Star Destroyer
AMT 1991 U.S.S. Enterprise Bridge [Installing Mods & Lights]
Finished Builds
Deagostini: R2-D2 [Never getting batteries]