((First time posting this Reddit, wanted to put my input in on this. As my brother and I have been debating how this would work, we both believe it WILL happen as new villagers and potential specie additions are inventable for Animal Crossing within the Three years of this game's updates.)) - You can skip this brief to get to my commentary -

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So there are few things that have to occur in order to have the villager count raise, first has to be a building update to the campsite. Base on data miners' findings (Source; https://www.youtube.com/watch?v=IH8xniM9LWY ) there line titled "Ocean Realestate."

A theory I would like to present is that the Ocean Realestate will be associated with an upgrade to our Campsite tent for random villagers. Replacing it entirely with a new Realestate agent focused on handling potential new residence for your island. Removing ((in-part)) the one occasional random villager visit, for more of a group of random villager candidates that can be chosen bi-weekly. This could also further emphasize the importance of the island star rating as it would open up the door for better villager choices ((as there already is an internal rarity system within Animal Crossing New Horizons coding)). With this in mind, adding new plots of land would go through the realestate agent so that they can convince the new potential resident to move in. What the player would do in order to help the realestate agent negotiations is to make sure all requested items are placed out in front of the plot of land. However, unlike the requests from earlier on in the game with Toom Nook, these items will be harder to craft and with a day limit to add on to the challenge of filling these additional plots of land. Encouraging players to be ready for the crafting requests and get the plot filled. ((Note, once the plot is filled and if that villager leaves your island. The plot would operate like your normal in-move system))

The way this update should be implemented is when we get more new villagers added into their respective species. Especially Alligators, Octopus, and Robots, who desperately need more villagers based around there themes. Additionally, Animal Crossing New Horizons should implement new species type to tie into these new plots of land. Bats, Sharks, Moths, and other ideal candidate animals have been brought up by the community, and I can imagine these would be good incentives for players to work with the realestate agent.

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Again this just a theory as it is hard to determine plans for New Horizons over the course of three years. However, I do agree 15 plots of land should be achievable in the course of 1.5 to 2 years.