You can apply a simple zoom by adding
"zoompan=z='zoom+0.001'"
to your video filters. It will, by default, zoom in to the top left corner slowly. If you want to do something a little more advanced, you can add the x and y arguments to start getting a zoom in other directions and variable speeds. An example of
"zoompan=z='zoom+0.001':x='if(gte(zoom,1.5),x,x+1)':y='y'"
would go to the top right and
"zoompan=z='zoom+0.001':y='if(gte(zoom,1.5),y,y+1)':x='x'"
would go to the bottom left.