Speculative Property

Fortify {1} (Attach to target land you control. Fortify only as a sorcery. This card enters the battlefield unattached and stays on the battlefield if the land leaves.)
Whenever fortified land is tapped for mana, it adds X mana in any combination of colors instead of the mana it would normally produce.
X is equal to the number of lands that have entered the battlefield since your last upkeep.