My first thought is: models with low production runs/desired, fanboyish type cars. But then, you can't really "know" this until years later when a legend develops around them. But at least, you can look at how many will be built and it's probably best to start there above anything else, because you know there will eventually so few vehicles out there in years to come (bonus: find one from manufacturer that makes shitty but desirable cars, because then you hope that plenty of them will disintegrate over the years. So....an Italian car. But then, such owners will typically baby their cars, so probably less prone to rusting away).
Has to be unique in some way. Typically, you look for cars that are the first to try something, and that turns out to be an example of the system (old bi-turbos with sexy sounding engines; first AWD rally car types that laid the groundwork for standard designs across platforms, companies, and even the industry, etc). Or, a one-off unique? I want to think something like the gorgeous 5cyl engine in the Audi S3/RS3 is going to be that, and relatively few are made, that it might be worth considering. But...really, there isn't much else there. it's an Audi and at this point, they aren't really that interesting from a collectible standpoint. The history that Audi will ever make was made back in the 70s and 80s. The RS3 market is saturated with tons of competitors, and there's no reason to think that any one of them will be super special--even though that engine is very unique...so what?
Or whatabout crossover technologies? A lot of supercars in recent years that are deploying the ICE/EV hybrid systems for incredible actual performance and almost fantasy power numbers....these cars instantly jump in value, because they are extremely limited (you have to get invites directly from manufacturers to own one; and sometimes you aren't even allowed to keep it, lol. I'm thinking La Ferrari, the P1, etc). Those are easy to pick as future collectors, but you also need a couple million, and more now, just to invest. Good luck.
I don't think you can look at any newish car these days, from a styling standpoint, and consider that it might standout a few decades from now as truly beautiful, representative of something from today....because cars more and more look the same due to safety and regulation/efficiency requirements. It's just getting harder and harder to make something look different. The curve will only ever track towards a very common design.
So, supercars, basically, and more those that might not even be street legal. That's all that's left. You have to pay to play, I think. Meaning, you can't look at something like a reasonable priced generation of M3, M5
today, and expect it to go up in value like some of the prized generations of those models have before it. I think it's too difficult. The only way to guarantee serious collector value in dinosaur ICE vehicles is to start at, say Ford GT tier and beyond, lol. ...Maybe some one-off Porsches or "special" 911s? (lol, what is a special 911, anyway?
This one, maybe?)