We discuss the design of Mopsa, an ongoing effort to design a novel semantic static analyzer by abstract interpretation. Mopsa strives to achieve a high degree of modularity and extensibility by considering value abstractions for numeric, pointer, objects, arrays, etc. as well as syntax-driven iterators and control-flow abstractions uniformly as domain modules, which offer a unified signature and loose coupling, so that they can be combined and reused at will. Moreover, domains can dynamically rewrite expressions, which simplifies the design of relational abstractions, encourages a design based on layered semantics, and enables domain reuse across different analyses and different languages. We present preliminary applications of Mopsa analyzing simple programs in subsets of the C and Python programming languages, checking them for run-time errors and uncaught exceptions.