Casting gimmicks are a bit out of hand in Hollywood right now. The Expendables might have been both the peak and the breaking point for the concept (while the all-female version of The Expendables is overkill), which isn't just about putting big names together but also requires some extra-textual significance to the idea. For example, the just-greenlit Grudge Match will star Sylvester Stallone and Robert De Niro as rival boxers returning to the ring, which is a blatant cash in on the kitsch factor of seeing "Rocky Balboa" (Rocky) and "Jake La Motta" (Raging Bull) go head to head.
De Niro has had a few of these, as they do tend to occur with older, iconic actors. One of the biggest was having him go up against Al Pacino in Heat, because in spite of them both being in The Godfather Part II they had never acted opposite each other before. Of course, they weren't together very long in that, and so someone had to go the extra mile eventually, and Righteous Kill wasn't worth the added joint screen time. De Niro's involvement in the Meet the Parents movies also led to the inspired casting of fellow major movie stars of the '70s/'80s Dustin Hoffman and Barbra Streisand as Ben Stiller's parents.
Inspired casting is what this sort of thing used to be known as. One of my favorites, if it counts, is Tony Scott's casting of Gene Hackman for Enemy of the State in a part that very much recalled his character from another surveillance--The Conversation -- especially with a very direct visual allusion to the 1974 film. More of a clever nod than something blanketing the entire production and then marketing of a film. Of course, Quentin Tarantino is the king, or maybe just the pioneer, of good gimmick casting, as he's been using actors for homage and star-text-exploiting allusions since the beginning. Joe Dante has also been fairly good at that.
Sometimes the gimmick is meant for ironic purpose, such as when Matthew Broderick plays a teacher in Election, and no viewer of a certain age can't help but be reminded of Ferris Bueller's Day Off. The original 3:10 to Yuma involved a villain played by usual good guy Glenn Ford and a good guy played by usual villain Van Heflin. And surely anytime Hollywood exploits the idea of a child actress "grown up" on-screen (some examples have been Alyssa Milano, Dakota Fanning and now Kristen Stewart), whether there's skin shed or not, is a kind of sorry subtrend.
Other less conspicuous instances include voice work, such as with the animated feature Shark Tale -- there's De Niro again, this time with Martin Scorsese -- and Small Soldiers, with its Dirty Dozen and This Is Spinal Tap reunions. Sigourney Weaver as the voice of the ship in Wall-E is also brilliant.
What are your favorite instances of gimmicky casting?