Hi Saurus!
Great first(?) map! I to draw inspiration from the artists you've cited here. I, who am new here and new to the finer subtleties of map making, shall try to offer some advice!

Empty spaces: Whether or not you make the quiet places more busy I guess depends on how you guys will use the map. One thing you could do is add simple illustrations or symbols such as trees, cacti, skulls etc to decorate and enrich the dull spaces. Maybe some mythical animals etc. I notice you have no forests indicated so maybe you could create some of those.

The latitude and longitude lines: I think you could keep things how they are now. It gives the impression this is part of a much larger area and you are looking at a projection that skews this part of the world so it is interesting. If you want to use the map to determine distances somewhat accurately a more regular grid would make more sense.

On symbols: Putting in some variety in the symbols on the map wouldn't be a bad idea necessarily I think. You can have a legend to explain what is what. A unique symbol for each event or location would be too much though probably.

On a legend: Simply, I think you could add a legend by extending the image to the bottom, top or even to the side if you wanted and create a legend outside of the borders of what you have. That is if you not confined to a certain proportions for this map. Actually, I'd suggest making the whole image a little taller so you could fit in a nice title up top. At any rate, the map is somewhat narrow as it is so the extra height would balance it out a little more and would not have to lose any map space you have already created.

Oh any potential faux-pas: I wish I could say. Certainly others may be able to point something out as many have more experience than I!