It looks as if this is a goer.
Deutsch/English/Denglish-mir ist egal! Not an issue- just trying to be compatible- just seems sensible to stick to the name prefix system, and its only abbreviations most of which are the same.
The path- There are two types of path (UK) footpaths for walkers and bridleways for walkers, horses and other riders- to a walker the change from footpath to bp is an important consideration- to a horseman it is the difference between allowed and not allowed. I suspect mountain bikers have a similar opinion- do we need to show both- how does this internationalise. I was thinking dark green/ light green but couldn't decide which way round- though if the distinction is not important it should default to dark as this looks clearer. (I'm thinking of printouts too.)
For gradient there seems to be several ways to proceed. As I have said, I was thinking of chevrons like the lock symbol.A double for 1:3 and a single for 1:5 (but this could be adapted meaning very steep and steep- which would vary in context). The German Bahn site has a section for Mountain lines where a spot height is given in a column to the right of the track. This seems simple, effective and could be used on footpaths. 
The colour of green needs to be stated in #rrggbb before anything else is done- reds and blues can be taken from canals and railways.
Glancing at the color-neutral link- is it more difficult for the author writing the router or just the icon maker? Can it be done in Inkscape?
I can do a few of the extra symbols fairly quickly if I am not distracted.