I don't think this can work with just these changes. Plots, character slots, consignment slots are the only monetized items the game has. Giving away a free plot will not help the game unless other charges take its' place.
Examples:
Want to build a t6 guild hall? $99.99 And this is just for a token required to complete the building, something similiar to the hiring permits. A Zoning permit or something.
Want to build a fountain? $9.99 (For the "Cornerstone")
Want to build a t6 shop? $39.99
Want to build a clock tower? $19.99 ("Clock mechanism")
Want to build a tree (any, each tree) $4.99 (for the "Seed")
... All these additional bits would have to be added to the building construction list. Require Ingenuity skill of 1 to apply. Or perhaps a completely separate list of buildings would be available on the free plots, those which require the bought pieces. And paid sub plots could have the normal buildings.
You see where it's going... This would be the only thing that could justify a free plot.