Hmmm ...
5,000-7,000 gallon shark lagoon.
Well you would certainly have several options. Depending on the dimensions, you could possible keep a couple of smoothhounds. Or possible set up a breeding pond for a couple of species of benthic sharks - i.e. Bamboos, Eppies, Horns or Coral cats.
But since your thinking -it will be about 5 years before you build this lagoon. As always - recommend taking the time to study and do more research. Maybe start eventually start smaller pond with one of the "aquaria friendly" species of Bamboos, Eppies, Horns or Coral cats. This way you'll gain some experience to help you when your finally ready to build the big shark pond.