Anyone used seitan or other more heartier vegan protiens in sandwiches?
No because I dislike sammiches but Future Lettuce's quick seitan has a tender, Chinese-style chicken texture that would go well in a sando:
https://files.catbox.moe/mxh23c.mp4
For a more complicated method that produces a ham-like luncheon "meat", I'd go for this recipe.
One filling for sammies or wraps that I think is pretty great is a veggie roast: get a tray, slap chunks of pumpkin, tomato, onion, garlic, and any other root vegetable or things like eggplant and capsicum and roast it all together with a coating of olive oil. Use as the base for a filling and add some herbs and leafy greens or other toppings like olives or capers as per your preference.
You can freeze this filling if you make a big batch of it ahead of time too.