All cars to a certain degree will become future classics with time and eventual demand outstripping supply, but ones which gain in value and be truely desirable well before just a time frame of 30 years as an example is a little harder to quantify, but I'll give it a shot.
One way is limited supply. Two is Motorsport heritage. Three high praise in the day from motoring press and the public wide making it desirable but possibly out with the finical reach of many young aspiring drivers. Those young drivers get to late 30's early 40's maybe a bit older now to reach a level of disposable income and hark after that car from their youth they couldn't afford or they actually owned and have overwhelming cherished memories of that time in thief life and want to reminise in it a little.
A series 1 Peugeot 106 Rallye is a 100% certainty to become a future classic, it already is. It was supplied in limited numbers circa less than 1000 and had a degree of Motorsport heritage from JuniorWRC as it was a homologation model built to compete within the context of the rules. It's a car that many myself included had when younger and I'd love an unmolested one to keep. Series 2 Rallyes are possibly better than ones but I don't think will gain as much long term.
My own Lancia integrale EVO is already a classic, it was pretty much a classic from the day they stop producing them. It was rare, it had massive Motorsport heritage as most successful WRC car of all time. BMW E30 M3's especially EVO models are the same, whereas the later E36 is in every single measurable way, power, speed etc etc is better but not a classic yet.
Some are also a regional thing, Ford have massive following in the UK so even cars that when new were not that good or failed miserably in the Motorsport arena they were conceived for are gaining in value. Fiesta RS Turbo, bit poor new, starting to gain credibility. Escort Cosworth, a modified Sierra had its pants pulled down in the WRC, but I'll still have one.
Conversely, Toyota GT4's, the only car to challenge and eventually surpass Lancia can be had for buttons, yet technically, especially the ST205 a car I've owned as well is far superior to the aforementioned integrale and Escort with much more Motorsport provenance than the Escort.
An UNO Turbo or MG Metro Turbo will never achieve the same as a Renault 5 GT Turbo or 205 GTi as they just were not as good when new, so less desirable new stands to reason less desirable now. Though all cars have their own fans.
The RenaultSport Megane R26.R will be a future classic, its only a few years old and already showin signs with very strong residuals caused by slow sales of already limited numbers as many saw it as 2 extreme as a daily car, but not extend enough as an all out track weapon compared to a Slotus or Caterham. It proved by dropping weight, softening the suspension consequently fitting a LSD you had a car that covered ground much much quicker than percieved more powerful cars.
As for Cento's, well the Sporting models will some day gain in value to those who look back fondly on what was possibly their first car or at least one of their firsts. Or even by those who maybe had a nonSporting model as couldn't afford to buy or insure a Sporting and promised themselves they would have one one day. It will just never get that cult following as it never achieved the Motorsport highs or public/motoring press accolades. In fact the Van Aaken turbo models will possibly by like early 500 Abarths or the other modified cars the name escapes me Gio something iirc.
You can never 100% accurately predict what is a sure fire classic as some just tick the buttons that can be over looked from time to time.