Hi there, and welcome to the Guild! In the future, you might be better off posting this sort of thing to the WIP section (probably to the World/Regional mapping) rather than the How Do I?? area, since people honestly tend to be a bit more active over there... at least in my experience. The How Do I?? section is usually used to ask specific questions about some mapping detail ("how was this compass rose made?"), but the WIP area is better when you want more broad advice and feedback on how to improve a map.
In any case, since I'm not a Gimp user (at least this looks like a Gimp made piece to me, correct me if I'm wrong) I won't be able to answer all of your questions, but I'll give you what I can.
First off, you asked about rivers, and honestly the best thing I can do is point you in the direction of the River thread over at the tutorials section. That one should give you all the information you need and then some.
As for the other elements, I personally like to plan for those in advance when I first start working on a map. I'll add some lines and rectangles as placeholders from the very beginning, so I'll know to leave room for them. I usually like making my map's layout as balanced as possible. For this one, I'd most likely add a border of some sorts, and maybe a title box in the bottom right corner and a compass rose to the left, and leave it there. Or perhaps a rather small legend box to the bottom left instead of the compass... not entirely sure. Either way I think the shape of the map makes is hard to add a large legend without it looking a bit unbalanced as a whole. I think the canvas size is fine already, and the elements mentioned should fit there well enough as it is.
I also think the map most likely is a bit larger than necessary - zoomed in at 100% you are much closer than you really need to be in order to see the details. This is honestly something I did too when I first started working digitally, my first maps were so enormous that my Photoshop was constantly crashing. These days, my maps are maybe 1/3 of the size they used to be, and it works just fine. It's a natural instinct to make the piece large, but in the end it most likely doesn't need to be half as large as you think. I'd recommend trying to downsize a flattened version of the map to 50% of the original size, and seeing what it looks like. You may find it to be plenty big enough.
As for the fonts and colors, it really is a matter of taste. I personally like using pretty clear and readable serif fonts, that still have a bit of character to them (Fogle Hunter and Humane Jenson are some of my favorites). But honestly, whatever pleases your eye. And if you are planning on selling the map, then whatever you have the commercial use license to. In this type of map, I'd recommend going with a clear, light color (white, or maybe a yellow-ish hue for example) and adding it a dark outer glow, stroke or drop shadow in order to make it a bit more readable. But as I said, this is just what I would do. There are no right or wrong answers here.
One thing I'd also recommend taking a look at, is adding some texture. Even a satellite style map like this can sometimes benefit from laying some textures over it - even just some aged paper ones. Give it a try at some point!
Hope this gives you at least some ideas. Keep at it!