Model-Based Engineering (MBE) and Product Line Engineering (PLE) have been combined, to handle new system development constraints like: increasing complexity, higher product quality, faster time-to-market and cost reduction. As observed by some authors, the derivation of a product from product line shared core assets has been insufficiently addressed and can remain tedious in practice. We cope with this issue focusing on having a flexible and reactive model-based derivation, and propose an incremental evolution by extension of the product line coupled with this derivation activity. Process and tools bridge the gap between Application and Domain Engineering introducing a semi-automatic feedback to benefits from the developments made in the Application Engineering. The approach is applied to a model-based product line dedicated to Class diagrams, and is tooled within the Eclipse environment.