Browsing on Reddit, I saw someone actually make a suggestion I thought was pretty good:

—Make all plots upgradable to all sizes.

I think that would be a good solution. Have plots keep their current Grade ratings, and when someone goes to purchase one, they can have the option to purchase the plot as a small plot, a medium plot, or a large plot. And then pay the respective price based off of the Grade + Plot Type they want to purchase.

Wishful thinking, hoping the developers decide to implement a system like this, but I feel like it could be a compromise to the various complaints surrounding the limited number of mediums/larges versus the large number of smalls there are.