The deployment of nanocavities may efficiently enhance the light-matter interaction for photonic components on chip. Three nanoscale cavity designs are investigated, including the one-dimensional (1-D) photonic crystal (PhC) nanobeam cavity, inline waveguide-integrated plasmon cavity, and square plasmon resonator embedded in the 1-D PhC nanobeam cavity (i.e., Combo cavity). The cavity performance, such as quality and Purcell factor, mode volume, and light-matter interaction strength, are evaluated for each structure for comparison. A deep subwavelength mode volume of 0.18 is observed in the Combo cavity, which exhibits an improved Purcell factor up to 428 and a 44 times enhanced interaction strength due to the compressed mode volume compared to the inline plasmon cavity. Thus, the Combo cavity shows promise of becoming a potential building block for active components of next-generation on-chip photonic circuits.