Cosmetic changes are fine, as long as they don't affect the in-game mechanics or your performance. Some examples of things you could do: Reskin the Shipka with an ACR, change trees to look nicer, changing cars to look nicer, etc. Some examples of things you could not do: Delete the tree image files for better vision, delete the gmap, make bullets neon colored for better dodging/visibility.
These are just generalities, and probably aren't 100% in compliance with Era's TOS.