The answer is...nothing! At least not directly. But indirectly it pretty much shows Nintendo's update...schedule? philosophy? idk what to call it♀
But basically SMP's biggest and most improved update came 3 years after the game was first released. SMP's update added online capabilities, new modes, and more options for multiplayer including online and local. It's pretty similar to the way NL was updated with the Welcome Amiibo(WA) update 3 years after it's release, after which, there were no more updates. The WA update for NL added more furniture, new modes, more functionality(like sitting on rocks for example lol), mini games, a whole new section of town, more villagers, QoL changes(better ways to move furniture around, placement for villager homes), and a lot more. If I'm not mistaken, both Smash and Splatoon are/have been updated in the same way, in each of their own release timelines, with Splatoon 2 getting tons of new modes years after release.
This is kind of a double edge sword for Nintendo. If you look at games across all gaming platforms(PC, Xbox, Sony, etc), most dlc is released within 1 to 1.5 years or so after the game hits the market, because most developers expect people will move on from their game after 3+ years down the line. Players get new dlc and QoL faster, but the game's life dies down quicker. Nintendo meanwhile is still pretty new to the live-update game(I mean relatively speaking anyway), and their main games are almost expected to have a long shelf-life. For instance, Breath of the Wild got its first dlc a few months after release. But the devs had so many dlc ideas for the game, they just scrapped the dlc plans for BotW and just decided to make a new game, BotW2 which according to them, isn't even close to ready. If they had stuck with they're original plan, and kept making dlc releases for the for the first game, we'd probably still be seeing BotW1 dlc years later after its first release in 2017.
Which brings me to AC. Now this is just a theory, and I'm only going by own armchair analysis. And what it looks like to me, if my theory is right, (and please keep in mind that I have no idea or any way to prove that it is, plus this is just pure speculation):
we won't be seeing anything close to AC 2.0 anytime soon. If I had to guess? I would say not until late 2022 at the earliest, or at the early 2023 at the latest. Looking at it now, I don't expect any major AC updates for 2021, not even for E3. And no I'm not trying to make excuses for the token updates we've been getting. We'll still get those types of updates with new items for events like summer, the holidays we already have, etc., for the rest of the year. But the big QoL stuff like Splatoon, SMP or NL got? All those didn't come until at least 2 to 3+ years after each of those games were out. I'm just speculating at this point that maybe NH is in the same boat. Because as popular as this game is, it seems a little weird that Nintendo would just cast it aside when New Leaf's WA update was very successful.
But basically SMP's biggest and most improved update came 3 years after the game was first released. SMP's update added online capabilities, new modes, and more options for multiplayer including online and local. It's pretty similar to the way NL was updated with the Welcome Amiibo(WA) update 3 years after it's release, after which, there were no more updates. The WA update for NL added more furniture, new modes, more functionality(like sitting on rocks for example lol), mini games, a whole new section of town, more villagers, QoL changes(better ways to move furniture around, placement for villager homes), and a lot more. If I'm not mistaken, both Smash and Splatoon are/have been updated in the same way, in each of their own release timelines, with Splatoon 2 getting tons of new modes years after release.
This is kind of a double edge sword for Nintendo. If you look at games across all gaming platforms(PC, Xbox, Sony, etc), most dlc is released within 1 to 1.5 years or so after the game hits the market, because most developers expect people will move on from their game after 3+ years down the line. Players get new dlc and QoL faster, but the game's life dies down quicker. Nintendo meanwhile is still pretty new to the live-update game(I mean relatively speaking anyway), and their main games are almost expected to have a long shelf-life. For instance, Breath of the Wild got its first dlc a few months after release. But the devs had so many dlc ideas for the game, they just scrapped the dlc plans for BotW and just decided to make a new game, BotW2 which according to them, isn't even close to ready. If they had stuck with they're original plan, and kept making dlc releases for the for the first game, we'd probably still be seeing BotW1 dlc years later after its first release in 2017.
Which brings me to AC. Now this is just a theory, and I'm only going by own armchair analysis. And what it looks like to me, if my theory is right, (and please keep in mind that I have no idea or any way to prove that it is, plus this is just pure speculation):
we won't be seeing anything close to AC 2.0 anytime soon. If I had to guess? I would say not until late 2022 at the earliest, or at the early 2023 at the latest. Looking at it now, I don't expect any major AC updates for 2021, not even for E3. And no I'm not trying to make excuses for the token updates we've been getting. We'll still get those types of updates with new items for events like summer, the holidays we already have, etc., for the rest of the year. But the big QoL stuff like Splatoon, SMP or NL got? All those didn't come until at least 2 to 3+ years after each of those games were out. I'm just speculating at this point that maybe NH is in the same boat. Because as popular as this game is, it seems a little weird that Nintendo would just cast it aside when New Leaf's WA update was very successful.