Prior to the @metamask/json-rpc-engine migration into the core monorepo, there was an ongoing initiative spearheaded by @rekmarks to simplify json-rpc-engine middleware, and make them async by default:

• Allow middleware to be async, deprecate createAsyncMiddleware json-rpc-engine#81
• RFC: Remove callback from next return handlers json-rpc-engine#82
• RFC: Remove next callbacks json-rpc-engine#83
• WIP: Improve middleware errors json-rpc-engine#84
• Add DuplexJsonRpcEngine json-rpc-engine#107

The team intends to follow-up on this to bring these efforts to completion.