If I was building a MUD or MMO from scratch today I'd try with Elixir (the language built on Erlang). Functional programming with hot code replacement, implementing the actor model for distributed, fault-tolerant, soft-real-time massively concurrent applications, does that sound similar to what you're looking for?
Growing that into a FaaS product would be hard though; Erlang processes all share a trust domain, from the infosec point of view, so containerizing each function/process for invocation lambda-style on otherwise shared infrastructure may prove difficult.
Each of my server processes in Go looks a little like an Erlang actor. There's a switch statement where a queue of input messages is processed, then each instance is run in turn. Instances also have a similar structure and maintain a "space" using an R-Tree. When it comes to efficiency, game loops are still a great way to do concurrency. Game loops per-instance allow you to sidestep many pitfalls of multi processor concurrency, much as per-process GC does with Erlang.
Growing that into a FaaS product would be hard though; Erlang processes all share a trust domain, from the infosec point of view, so containerizing each function/process for invocation lambda-style on otherwise shared infrastructure may prove difficult.