R37 Roasted peppers make delicious sandwiches, they can be paired with egg, with or without cheese, or anchovy paste if you are not giving up fish as well.
Avocado, sprouts, tomato, cheese, and lettuce can be a lovely sandwich, especially in warm weather.
Falafel, and Middle-Eastern salads all make satisfying sandwiches. Any stir-fry vegetable combo with oil, garlic, and seasonings can go into a pita, or a tortilla to make a delicious sandwich. You can keep the stir-fry in the fridge for a few days, to pull out as needed.
I'm also very keen on small radishes sliced thinly on a buttered baguette; you could easily add lettuce, sprouts, or avocado to make a more hearty sandwich.
Though I'm usually not one to recommend a lot of soy-based synthetic meat substitutes, I've always enjoyed a fake bacon sandwich!
I'm definitely going to try the Beyond Meat mince, I think there would be endless ways to fix that, other than burgers.
Creamed mushrooms (you don't need cream if vegan, just a vegan margarine and a bit of flour) on toast can be a simple enough sandwich, get fancy with white wine and garlic if so inclined! Cheeses such as Gruyère, Emmental, or Lustenberger "1862" go well together, a bit of Parmigiano-Reggiano can pair well too. (or a combo!)
Cucumber sandwiches are always a warm weather classic, and need not be dainty tea sandwiches. Make them as thick and crusty as you like: spread cream cheese mixed with chives, or fines herbes, or simply fresh Thyme by itself. Watercress is a nice addition as well.
So many ideas....
