The halfway "best of both worlds" is impossible to attain, in my opinion. As far as I understand it, dynamic typing (meaning, extensive usage of runtime type information) has unmatched flexibility. Static typing (meaning, extensive analysis of syntactic types at compile time) completely prevent large classes of errors.
If you want a middle ground, you may lose some of the flexibility, and you still won't be able to prove as much as a full static type system. In the end, the "best of both world" could rapidly become the worst of both worlds.
As I see it, we have to compromise. When you design a type system, you want to maximize 3 virtues: flexibility, simplicity, and error sensitiveness. Alas, of these 3, you can only have 2. Dynamic type systems typically are simple and flexible, but hardly prove anything (which explain why unit tests are so useful). Advanced type systems like Haskell's are quite flexible and prevent many errors, but they are complex. Others, like Java's, are simpler but not as flexible (nor as error proof). And of course you have horrible type systems, like C++'s, which lacks all 3 virtues.
I find that with a good, type-aware compiler such as SBCL, Common Lisp hits a really sweet spot on the typing issue. It catches a crapload of mistakes at compile time that would be runtime errors in Python (the language, incidentally the SBCL compiler is also named Python) and most other dynamic languages. The type-language of CL is also more expressive than anything else I have seen, featuring unions, intersections, predicates, subtyping etc. But most importantly, it stands in the background, and doesn't stop you from executing programs that don't conform to some preconceived notion of what typing should be like.
If you want a middle ground, you may lose some of the flexibility, and you still won't be able to prove as much as a full static type system. In the end, the "best of both world" could rapidly become the worst of both worlds.
As I see it, we have to compromise. When you design a type system, you want to maximize 3 virtues: flexibility, simplicity, and error sensitiveness. Alas, of these 3, you can only have 2. Dynamic type systems typically are simple and flexible, but hardly prove anything (which explain why unit tests are so useful). Advanced type systems like Haskell's are quite flexible and prevent many errors, but they are complex. Others, like Java's, are simpler but not as flexible (nor as error proof). And of course you have horrible type systems, like C++'s, which lacks all 3 virtues.