The problem with predicting the next big thing or gambling on a continued rising prices is that the buying public are notoriously fickle and if/when government policy shifts away from petrol cars to EV in a big way (the current trends are just a dipping their toe in the water) then combustion vehicles will see bigger increases in taxation and fuel duty. An E46 M3 is going to be worth about £3.50 in 10 years if fuel is £2/litre and tax is £1000+ a year. Most enthusiasts of the 1990-2010 period of cars are old enough to remember when they were not that expensive. Gibbo, I remember your minty mint CSL at the time was ~£25k? And it wouldn't have been all that old and now a grotty one is £40k and that's been the case for ages.
I just don't think there is a big enough pool of people willing to pay silly money for a lot of these cars that won't just run a mile as soon as they start costing serious money fuel/tax/servicing and begin to depreciate heavily as part of that. Driving fast/loud/silly petrol cars will gain a stigma similar to smoking has now (which used to be the height of cool) and the vast majority of investors who bought up old crap because their money was doing nothing in the bank will move on to something else. Leaving this to return to a more normal state.
A good example is the 996, given the quite frankly absurd prices of most 911s, there's no sensible reason why 996s should be languishing at £10k. Early ones are really old and analogue just like a 993/964 which cost 5 times as much now. Ooo boo hoo the engine is cooled by water and it's got funny headlights. Totally irrational and given how unloved 964s were until 6 or 7 years ago it it just shows how silly things have got.
You have to remember though as time goes on, the vehicles naturally reduce in numbers, written off, broken down and can't be afford to repair so scrapped, turned into track/race cars and or stolen.
When I got my 172 Cup there was 1000ish taxed and on the road, I paid £1100 for it, when I sold it over a year later, albeit fully restored there was 650-700 on the road and because it was an original example, 1 previous owner and relatively low miles I sold it for £6500 and they continue to go up in value as the numbers drop on the road and those left with unmolested clean original examples the value goes up.
However many E46 M3 are on the road today you can sure bet that number will half or more in ten years time and the ones left standard, clean will be absolutely minimal.
Regarding the CSL mine is still on the road, taxed and MOT with 59k miles now, that car will be worth 60-80k being a low mileage example in good condition, the sub 30k mile cars have traded hands closer to the 100's, in another ten years there will be less CSL's on the road, not so many because most owners do seem to have garaged them, for example my old CSL in the past 5 years has done 5000 miles so clearly the current owner is preserving its mileage and either betting on its value going up or its become a dedicated weekend / track car.
Yeah it will be bad if fuel becomes £2 a litre, tax £1000 but I guess another way I look at it, I pay £570 tax now and £1.50 litre now, if it goes up well it goes up I am not going to stop enjoying cars. What I may do is get a boring hypersonic fast EV to save money, and then on weekends have something truly proper special, that sounds awesome, chokes people on its fumes as it drives by and thoroughly enjoy the thrill of not being the lemming and following a trend, fuel combustion cars will still be on the roads in 50 years time, they will just be a minority but will still be running. People always want to be different and well with EV its just a sheep thing, they are all the same, all sound the same and all totally souless in the way they drive.
As to regards to 996 I guess I am of that camp its got funny headlights, I just find the 996 not a nice looking car but also having driven 996 Carrera's I am sorry but for me they drive not a patch on the 997 series that came after, the 997.1 if anything was the last and best of Porsches totally analogue cars, the hydraulic steering hit a pinnacle height of feeling, the looks and size was spot on and they were lighter than the 996's, there only downside was engines made of chocolate but judging by values it does not seem to hinder them much, looks, noise wise I prefer 997.1, but to drive a 997.2 GTS is fantastic even if the steering is not quite as crisp but still fantastic. The newer 911's though although excellent did grow in size considerably but if I was to buy a GT3 today I'd buy a 991.1 GT3 RS, I just love how it looks and though a 997.2 GT3 might be worth more in future and though a 991.2 GT3 manual could really tempt me if the clutch weighting is suitable for daily driving.
Any car sold in lower numbers or produced that is special and the last of NA for example the clean examples will generally always be worth more money as they become rarer, particular if they were the cars reviewers raved about.