Several recent proposals have shown that re-architecting the host network stack can significantly improve throughput and reduce tail latency. Yet these designs remain confined to user-level stacks or invasive kernel forks, both of which are impractical for production deployment. We ask: Can host network stacks be made sufficiently extensible to support the incremental deployment of new designs? To answer this question, we identify the key capabilities required by such designs and propose the design of FlexNet, a framework that enables developers to implement new network stack designs as extensions to existing production network stacks.