Upcoming 'Technical' week (database)

As I wrote on our Trello board, we have two cycles of development: Gameplay weeks and Technical week.
While gameplay weeks take 3 of 4 weeks in a month, technical week takes one. 3:1 is good ration for technical vs gameplay features.
Of course, it is not that bold and technical features affects gameplay and gameplay features have technical and so on, but you got the idea - keep the balance between gameplay and engine/ GUI/ data works.

Concerning upcoming technical week I have one BIG work to be started. Game should be able to keep world data in a client-server database. It doesn't mean, it should be on a external server, of course, but the game should work with it this way.
We need it for:

• game save / load.
• quests. Game can issue a quest, using world data.
• real server routines. It would be nice to allow other players' games to affect your gameplay.

It means that main game may:

• Upload its current state to the database.
• Load specified local level from it.
• Get any information game needs from it. (Do specified level has necromutans? Its seed? or specified workshop? etc)

The most important part, main game doesn't care how database going to do it. Saves it to local drive, keeps in memory or uploads to fallenroguelike.com server.
For the game it would be 'a server'.
The development of game database can't be delayed for any longer.
Nearly all our upcoming gameplay features need it. It is as important as implementation of widgets.

We start discussing its architecture and prototyping. If you have any interesting suggestions, articles to read or very good with databases, you are welcome.
Any advise matters!