I love the notion of co-located processing and data, but most of the data stores dont quite embrace it. Well actually Riak does quite well if you're fine with a dynamo like kv store. The riak library is actually pretty nifty for distributed computations. But with Basho out of business it's harder to justify. Plus KV stores really aren't as usable as good ole sql. In that realm there's ActorDB, but I've not been able to figure out how to run it inside my own beam cluster rather than standalone.
Hadn't heard of ActorDB. Interesting. But is it alive? Seems inactive.
Horizontal scaling of the DB seems like a problem in many architectures. For SQL I'd be looking at CockroachDB as it is Postgres-compatible and is built with scaling out in mind. Haven't tried it though. Most of my work hasn't needed that for the DB recently.