Microarchitectural enhancements that improve performance generally, across many workloads, are favored in superscalar processor design. Targeting general performance is necessary but it also constrains some microarchitecture innovation. We explore relieving this constraint, via a new paradigm called Post-Fabrication Microarchitecture (PFM). A high-performance superscalar core is coupled with a reconfigurable logic fabric, RF. A programmable interface, or Agent, allows for RF to observe and microarchitecturally intervene at key pipeline stages of the superscalar core. New microarchitectural components, specific to applications, are synthesized on-demand to RF. All instructions still flow through the superscalar pipeline, as usual, but their execution is streamlined (better instructions per cycle (IPC)) through microarchitectural intervention by RF. Our research shows that one can achieve large speedups of individual applications, by analyzing their bottlenecks and providing customized microarchitectural solutions to target these bottlenecks. Examples of PFM use-cases explored in this paper include custom branch predictors and data prefetchers.