Now, with rising beef prices, it looks like a possible solution is eggs.
Out in Malaysia, McDonald's has started offering egg as a beef patty substitute on their value menu with the new Beef & Egg and McEgg Deluxe.
The Beef & Egg is basically a McDonald's Hamburger with an Egg McMuffin-style egg on top of the beef patty, while the McEgg Deluxe features an egg, cheese, lettuce, tomato, ketchup, and onion. Both are offered as part of a "McValue" combo deal with medium fries and a drink for 5.95 Ringgits (~$1.92 US) around lunch and dinner time.
Now this is not the first use of the McEgg moniker; the McEgg was first introduced in Malaysia back in 1987, but was eventually discontinued. McDonald's India also introduced a McEgg last year as a value-priced poached egg sandwich: