A lot of people want to build their own houses from scratch for which they will need a lot or plot of land. Build a global app which does the following:
Curate listings of plots currently for sale
A deed search for an empty piece of land to find out who the owner is (and if owner is willing to sell)
View zoning laws around plot

Monetization methods
Sell ads to architects, builders and building supply companies
The deed search is a premium feature with a premium price for every search