Also, I think people are overrating the two driver mechanic from Double Dash. Having one person driving the kart and the other one shooting items made for an interesting dynamic for sure, but it wasn't something that couldn't be accomplished by one player. Choosing two characters also gave you access to two unique items, but in hindsight, the unique items weren't a good idea because it created a power creep between characters. There was little reason to choose someone with a mediocre item like DK and Diddy over someone with an awesome one like the babies. I still enjoyed the feature, but I can see why they got rid of it. They took some of the item concepts and gave them to everyone rather than limiting it to just a couple of characters, like how the Chain Chomp item pretty much became the Bullet Bill.
I personally feel that stuff like anti-gravity tracks added more to the gameplay without detrimenting it.
I would say Double Dash was defiantly the black sheep of the bunch. It was daring and tried a lot of new things. Some were good. I'm glad for Mario Kart Double Dash because features that are for the better of the franchise may have never made it into future Mario Kart games without this installment. I would say that the ability to choose any kart regardless of character was a great addition. It eventually evolved into being able to choose the wheels and the glider which is even better. All cup tour was a great way to test the player's overall skill. I do wish it would return to future Mario Kart installments, but I can understand why it was removed (Retro Cups and DLC Cups). The additions of Shine Thief and Bob-omb Blast as separate battle modes had me and my friends playing for hours by the way, and we were having a literal blast with the additions coming back to Mario Kart 8 Deluxe.
That being said though, it's still the black sheep installment. Some of the new mechanics weren't for the better which hurt the game a little bit. The concept of double racers is nice on paper, but it creates a power struggle. For one, me and my friends are super competitive players no matter what the game is. We all figured out who the heavyweights were and pretty much all wanted to play them. Unfortunately, there were only three starter ones. Donkey Kong, Bowser, and Wario. Even if everyone only took one heavyweight, it still wasn't enough for everyone to get what they wanted. Also, for the groups of people who prefer lightweights, there were only eight in the game, and two were unlockable. Everyone would have to take two light weights, and that might not be enough to guarentee everyone gets what they want. On top of that, characters had special items. This not only unnecessarily limited the heavyweight class to taking a character with at least one of three items as well as the lightweight class to five out of those nine items entirely, but this created a power struggle over who can get to the best special items first. Every person in the room wanted that damn Bowser Shell so much so that we had to ban the Bowser and Bowser Jr combination over this. Nobody wanted to get stuck with Donkey Kong if they even got a heavyweight kart at all. For obvious reasons, when I inevitably unlocked Petey Piranha and King Boo, all hell broke loose in the bickering over who will get to take one of those two every time we played.
Don't get me wrong. We need these types of games to exist. Double Dash tried a lot of new things, and while not all were good, it set a positive precedent for future games. Coming back around to the original thread at hand, I don't think Smash Bros has truly seen any Double Dash like game yet. Most games that came after Smash Melee barely changed anything significant, and the ones that could be big changes (such as Custom Moves or Final Smash Meter) were togglable features and got ignored by everyone completely. The last time Nintendo really tried to make a game vastly different than its predecessors, it got ignored and was nearly completely forgotten about for that reason. I still have nostalgia from playing Smash Brawl, but it really wasn't a step in the right direction. It seems to me like Smash Bros hasn't been going anywhere in terms of new ways of innovating the fighting.
Side question. How did this turn into a conversation primarily about Mario Kart Double Dash?