I also think it makes sense to explore additional Apache Geode features and capabilities that may be possible to wrap with a Reactive API (i.e. Reactive types and Reactive behavior (where possible)), including, but not necessarily limited to:
Function Execution (by first adding support for Streaming Results)
Function Executions is perhaps less apparent in its path to Reactive, but if there were a fire-and-forget (i.e. async) method to invoke a Function, then it would be possible to execute a Function in a Reactive context. I think wrapping the ResultCollector would give you that async/Reactive-like behavior. Additionally, a StreamResultCollector implementation could be wrapped with a Flux.
Additional thoughts and ideas will be added in comments