The software industry has to offer increasingly individualized software for a large number of platforms. In a constantly evolving technical context, the appropriateness and the profitableness of a software has to be ensured earlier, before most of the costs have been incurred and before most of the risks have been taken. Feature-Oriented Model-Driven Development (FOMDD) is a promising paradigm to tackle the issue of developing software variants when multiple platforms are targeted. However, because of its model-driven fundament, FOMDD suffers from limited capabilities regarding model execution and early validation. In this paper, we present CrossFabrik, an approach for the design and the early functional validation of feature-based applications. This approach allows the live debugging and editing of the underlying models during a simulation without being forced to stop and restart a validation process. Such an approach relies on the reflective capability of the development environment. An implementation of our approach within Pharo is also presented.