This could be used as a way to seek and destroy hardy parasites (for example Leishmania) or bacteria that evade the immune system (like H. Pylori). It could also be used to deliver organism-wide gene therapies without encountering resistance from the immune system.
We would take a suitable type of innate immune system cell (possibly neutrophils, macrophages, or NK cells) and engineer it to indiscriminately deliver nucleic acid payloads into any type of cell it encounters - be it human or pathogen.
The nucleic acid payload then gets transcribed/translated into a Cas9 type of enzyme that looks for a specific sequence belonging to a pathogen. If a match is found the cell gets destroyed. Otherwise, the enzyme degrades and the cell lives on as if nothing happened.
A suitable nucleic acid delivery method could be copied from viruses. The delivery cells could multiply into millions of copies via clonal expansion. In terms of function, this would be a chimeric cell - part viral, part B cell, part macrophage, or similar.
What would it take to make something like this work?