This already exists: in "Trip options", you can toggle between "Best route", "Fewer transfers", and "Less walking", as well as allow/block "Ride services" like Uber as a transport option.
How well these options work is another question, but "Fewer transfers" does seem to allow more walking. And of course you can straight up ask for walking directions instead.
How well these options work is another question, but "Fewer transfers" does seem to allow more walking. And of course you can straight up ask for walking directions instead.