Just want to make sure you have the basics down for caring for them before we move on to pond ideas. Are you using dechlorinator in the water? What are you feeding them? do they have a heat bulb? What about UVB? I am sure I missed some important factors as I have never cared for turtles, but these are a few things that are common needs for many reptiles.
As far as pond ideas go, how large are you thinking? somewhere in the 300 gallon range, or somewhere in the 3000 gallon range? After we have size figured out, the filtration would be a next step. An upflow wetland filter would be a great option on any pond, and there are some great builds and articles on here about them. And finally what types of waterfall, fountian, or other features were you thinking of adding?