I think the sought after cars in the future will be the sporty and less common variants of older popular chod. Most froggie stuff will never be worth much.
So here's my list until I think of some more.
Vauxhall have quite a few potentially interesting cars that are going to have the values skyrocket in time.
First are the VXRs, for obvious reasons. The Meriva is a niche one. There's also the Astra G and Zafira A GSI, in the generation before the VXRs, along with the Corsa C 1.8 SRi, and the Astra G Coupe.
The Ford Fiesta Zetec-S will be an interesting one (the square light one not the original that's already OSF'd), sitting one below the ST. We don't really get cars like that any more, now you just stick a bodykit on a 1.0 Ecoboost and be done with it.
I bet Fiesta STs with the optional stripe package will have a bit of a premium over those that don't, too.
Thinking about it, the 'sporty' model Transits will probably have their place in the future too, if they're factory ones and not just boggo ones barried and blinged to within an inch of their lives.
I think the 2001-on Civic Type-S will be an interesting one too, being a slightly detuned five-door CTR (IIRC they have 160bhp). Completely unassuming cars aside from the '2.0' badge on the grille...