We've experimented with vegetarian cottage pie over the years and we make it differently pretty much every time.  A few days ago my lovely husband made a particularly good one by:

Mmm, cottage pie

Chopping a couple of onions, frying them for a bit, adding some chopped carrots until softened, then some frozen soya mince, some frozen peas and sweetcorn, a tin of baked beans, some gravy granules, salt and pepper, topped with sweet potato mashed with more salt, pepper, mustard, and a bit of cheddar cheese on top.  Baked for about 20 minutes on 200 degrees C.

Other things that can be nice in the filling:  lentils (particularly dark lentils), black-eyed beans, kidney beans, marmite, finely chopped celery.  It's basically quite hard to get pulses + soya mince + gravy sauce topped with mashed potato very wrong.

Cottage pie released to a plate

Cottage pie still in its cage