I understand @mattclarke's comment is on the amount of available working spots per land, whereas mine was on the max cap of each spot. Likely some sort of both could be implemented.
For example: You have a rare field with a lvl 3 farm built, this gives you 3 working slots, 1 for summoner and 2 for monsters. Then your rare summoner lvl 3 or any higher will be producing 30 wheat per hour. When you upgrade farm to lvl 4, you open up another working spot and the lvl 4 summoner (or any higher) will produce 50 wheat/hour.
Another possibility is that for the upgrades of the buildings themselves you will also need to assign summoners and monsters and have those "builders" work there for x amount of hours. That is besides the materials and DEC/SPS cost.
Also another beauty of land is that the number of working slots can be adjusted with time, that is when the number of players and new pack extensions grow (more cards in the game), more slots become available per land. For example maxed out building on a land has 6 slots in 2023, but 8 slots in 2024, 11 slots in 2025,...
Will definitely be interesting :D