Have you considered using actual vector graphics software? I find Inkscape immensely useful in mapmaking and I aim for a similar fantasy novel style which I find Inkscape particularly suites.
Well, the first thing to consider is, what do you really need to indicate for the map to serve its purpose? Cramming to much information is bad. If a feature set isn't included you don't need to worry about how to symbolize it and you'll have more flexibility in selecting effective symbology for the features that are important.
For forests, there are two reasonably common classes of symbol used on this kind of map when they show forests: Individual trees, or boundaries. Individual trees are just that, lots of tree symbols which may be packed tightly together in which case you need to get overlapping right, or more spread out. The tightly packed variant often simplifies the representation of the trees, particularly the rear trees. Boundaries are more complicated. The simplest is just a line, although a line with a scalloped edge is more common. Representation of trunks under the leading edge, or individual trees making up the edge are also common. clearings and similar details are often interspersed in the interior.
For mountains there are similarly two styles I've often used. Discrete overlapping side or elevated mountain icons, or "marching caterpillars" of hachuring. A third method, is to represent the mountains not as individual icons but as an integrated symbol for the entire range using a similar elevated view but I haven't used this method.
Grassland, when I've needed to indicate it, I've usually shown using tufts of grass. For savannah, I add a few trees spaced among the grass if I'm using discrete trees for forests (which I usually am if I represent grassland with discrete tufts of grass) I particularly put a few trees near any rivers even if it's otherwise pure grassland or scrubland rather than savannah.
I've got some slightly more detailed notes on how I put this sort of thing together in Inkscape which you may find useful, possibly even if you don't use Inkscape. http://www.cartographersguild.com/tu...ng-basics.html