Hi @kuku_super, thanks for reaching out!
An async API on the C driver has been considered before. It is not outside the realm of possibility. Notably, the investigation in CDRIVER-27 concluded the current multi-threaded API could maintain throughput well as the number of application threads increased.
Do you have a use case in mind for an async API? Has the number of threads spawned by libmongoc been problematic?
Sincerely,
Kevin