Understanding evolution of plant immunity is necessary to inform rational approaches for genetic control of plant diseases. The plant immune system is innate, encoded in the germline, yet plants are capable of recognizing diverse rapidly evolving pathogens. Plant immune receptors (NLRs) can gain pathogen recognition through point mutation, recombination of recognition domains with other receptors, and through acquisition of novel integrated protein domains. The exact molecular pathways that shape immune repertoire including new domain integration remain unknown. Here, we describe a non-uniform distribution of integrated domains among NLR subfamilies in grasses and identify genomic hotspots that demonstrate rapid expansion of NLR gene fusions. We show that just one clade in the Poaceae is responsible for the majority of unique integration events. Based on these observations we propose a model for the expansion of integrated domain repertoires that involves a flexible NLR acceptor that is capable of fusion to diverse domains derived across the genome. The identification of a subclass of NLRs that is naturally adapted to new domain integration can inform biotechnological approaches for generating synthetic receptors with novel pathogen traps.