Supernatural's famous Impala was originally to be kitted out with all manner of paranormal gadgetry - like a James Bond Aston Martin specifically for ghostbusting. When Misha Collins first appeared as Castiel in Supernatural season 4, fans quickly came to consider him the third lead alongside Jensen Ackles' Dean and Jared Padalecki's Sam. In truth, that spot was already occupied by the 1967 Chevrolet Impala the Winchester brothers inherited from their father to ride across the country in pursuit of spooky happenings. A character in its own right, Supernatural's Impala was even afforded its own dedicated episode, which is more than most human cast members can claim.
In terms of mystical modifications, Sam and Dean's Impala is surprisingly lacking. The bodywork is rigged with hex bags and warding to avoid being afflicted by enemies (not always successfully), and there's a devil's trap to guard the generous store of weapons stored in the trunk. These modest additions aside, Dean's Chevy is, essentially, just a regular car - a reliable workhorse to carry the heroes from one calamity to the next.
Strangely, this wasn't always the case. In a 2019 Reddit Q&A session (via Cinemablend), Supernatural creator, Eric Kripke, revealed major Impala changes were considered for the show's pilot episode. As per Kripke's description, the Impala's dashboard would've been fitted with custom switches. Hit one, and oil spurts from the back. Flip another, salt releases to trap any vengeful spirits that might be tailing. By the creator's own admission, the idea was swiftly dropped for being a little bit rubbish.
And it's certainly difficult to argue with Kripke's reasoning. The appeal of Supernatural's Impala rests in its rugged charm. The automobile runs on regular gas, gets beat up, needs a little love to run properly, and holds plenty of fond Winchester memories. It's not some miracle car like Knightrider's KITT (well, maybe once), or a tank like Bruce Wayne's Batmobile; it's a hunk of steel and an American engine valiantly cruising through all manner of dangerous and fantastical situations. To compare it to an animal, Supernatural's Impala would be a loyal Labrador, not a bloodthirsty hellhound capable of tearing flesh from limb, and this is precisely why the car remains so popular, both with fans, and with Dean Winchester himself. The brothers may be the destined vessels of archangels, but the Impala is their plucky underdog companion.
Packing out the Impala with gadgets also makes very little sense in Supernatural's context. Sam and Dean possess skills aplenty, but they aren't exactly on Q's level when it comes to technology, nor do they have the wealth of MI6 or Wayne Enterprises to indulge in such grandiose modifications. Forged credit cards likely don't stretch that far. It's also tricky to envision any situation where shooting salt or oil from the rear of a car would be practical. On precious few occasions has a ghoul or demon actually ran after the Impala on foot while the Winchesters drove away, and trying to position the car so that its salty blast hits the enemy would be farcical.
But most importantly of all, a James Bond-style Impala would've undermined the Winchesters' rugged hunter-ish qualities. Cars with gadgets belong with the British Men of Letters, not the beer-guzzling, pie-munching road-weary hunters, and while the Winchesters' arsenal certainly gets more sophisticated through the years, the Impala is perfect as it is in Supernatural season 1.