At minimum, it's _crazy_ to me that McDonald's does not have a veggie burger on the menu. Why?
IMO McDonalds have excelled when it comes to Coffee and marketing the McCafe brand and I don't doubt they could do the same thing with "healthy food choices" I just think the hype around healthy fast food has died for now and McDonalds is not willing to push it until the public demands it again.
That part is OK. I'd be really annoyed if my beef tasted like chicken or vice versa.
The problem is finding something quick and cheap that you can slap into a bun in seconds and sell for a couple bucks that tastes good. I don't care if it "kinda sorta tastes like beef sorta" but I want it to taste good.
The closest I can think of for vegetarian sandwich off the shelf would be some veggie subs I've eaten at fast food sandwich stores and the closest I can think of in theory would be some kind of marinated veggie kabob turned into a sandwich. The labor costs would be killer.
In an alternative direction, I like spinach tamales but I have no idea how to "sandwich-ify it"
I want to eat tofu that kinda pretends to be beef about as much as vegetarians want to eat thinly sliced pork ears that pretend to be lettuce. Make it taste good, not taste like a clone.