I know that it is possible to create a IndexedDb at client side, but I was wondering if it possible to create a key/value store server Side. In that case we could use that db as a cache server or even as an elasticSearch server, or maybe replace a SQL or NoSQL database
You can use any database on the server where a Dart driver is available (not so many yet, but in the future there will be support for most mainstream databases.
take a look at