Been lurking here for awhile, signed up for this. If you're really serious about making this a fully fledged, modern client, you should look into implementing physically based shading and potentially a real time gi technique, several of which (such as light propagation volumes and radiance hints, the latter of which tends to yield superior results) are relatively simple to implement and work wonders for realism.
There are papers scattered throughout the net for PBS, but a great resource may be to look through the UE4 or Unity 5 shader code to get a feel for how they're doing things, especially since you said you're already using a deferred renderer (I would hope deferred shading rather than light prepass since, in my experience, the former tends to be more efficient especially with modern gpus).
As for GI, there are a number of papers here talking about various techniques http://graphics.cs.aueb.gr/graphics/research_illumination.html , many of which include code. If you do end up wanting to implement radiance hints, a good example of it being used effectively is the open source Tesseract game/engine. I haven't personally dug through the code, but it uses a deferred renderer + radiance hints for GI and looks pretty damn nice. More on that here http://tesseract.gg
Also, screen space reflections, which there are plenty of resources around (including a bunch of source code) to get working.
All in all, you've done great work so far - color me impressed. But you could go even further

I second the motion for making this into a community project.