Watership down takes place in the English country side, southern England to be exact, and consist of multiple environments throughout. The book has rolling hills, lush forests, ditches, rivers, farms with humans and more all in which I shall include in the book. Implementing these in the book can add mechanics and depth to the game, hopefully enhancing the gameplay. The River could provide an obstacle for the player which will have to find a way around it, or implant some kind of simple puzzle such as trying to find a log in order to float to the other side, maybe even some stealth element such as sneaking onto a boat avoiding humans trying to find a way on. If you get caught the only punishment will be getting out back to the start of the stealth segment, in order to make it less annoying.
Forests could have predators which allow for greater enemy diversity, and with a day and night cycle, traveling by night makes you prone to owls which are increased in numbers in forest but with a trade off of less predators. Maybe by implementing some camouflage you could avoid many of the owls which gives the player more of an incentive to travel at night rather than it just being a gimmick.
The second Warren with Cowslip had a few traps which humans set up, so navigating environments like these will have to be approached with caution and wit. Implementing an upgrade later on in the game would be wise, allowing the rabbit to destroy traps in order to traverse environments quickly and stop them being tedious in the later game when they have been crossed multiple times.
Farms could allow for players to steal items such and carrots and other resources which allow for the upgrade of abilities, but has the risk of getting caught by a human or cat. If caught the player will lose items.