Maybe a bike / propeller powered glider would be cool. You could traverse the map silently in the air and land in and take off in open areas ( fields , roads etc). This could be an alternative to fast travel. Also, I heard you were considering adding wildlife , but I think that would dilute the menacing atmosphere of the game. Think about it, if this invasion actually happened in real life, those machines would have killed off the animals long ago, so the fact that there are no animals around makes the menace all the more real.
Ps. I hadn’t heard of this game 'till I saw it in a youtube new games video . I have to force myself to stop playing it, easily one of my favorite 2 games now. Awsome!!