If I might do a bit of speaking up for the sometimes despised-and-overlooked -- viz. here, in the often-regarded-as-dull more easterly parts of England where I was born and grew up -- something I'd love to see (it might well at this stage require magic, rather than a mere £30 million), would be: the Nene Valley Railway extended from its present western limit, the five or six miles or so onward to the station, in its proper and as-was site, at Oundle. For a good many years after the 1964 closure of the Peterborough -- Northampton through route, the line remained in use at the Peterborough end, as far as Oundle; for freight, and for beginning-and-end-of-term school trains serving Oundle's well-known public school. It's a beautiful little stone-built town: deserving in my view, more attention from visitors. And from my, admittedly verging-on-bigoted, type of railway-enthusiast POV -- this would make a preserved line of worthwhile length, and running from "somewhere to somewhere", rather than the present in my view rather absurd "somewhere to nowhere".
I do recognise reluctantly (having had this "take" expounded to me, by volunteers on this very line -- and as touched on by the OP) that re things pragmatic: very many of the visitors on whose fares preserved railways rely, prefer a "short-and-sweet" steam experience (say half a dozen miles each way) to a longer one -- and don't give a damn about sentimental old-style railway enthusiasts' concerns over "somewheres and nowheres".
