So are saying to add like a shallow bluegill pond just north with overflow into the bigger pond?
Yes.
If you want to build your main pond "above grade" to create the additional depth, then you will need some type of water source. IMO, if you are paying some extra money to create a water source, you might as well get some extra value for your property and fishery goals by having that water source also be an asset for some other needs.
