When they get around to Expendables 4 - because I'll believe they'll stop at 3 precisely never - which action stars are left to add? The third one is going to have Harrison Ford, Wesley Snipes, Antonio Banderas, Mel Gibson, after all the ones they've used in the first two movies. I guess they could dig up Charles Bronson and Lee Marvin, just run a current through them periodically to make them move semi-convincingly. Or put some headphones on them, see if Weekend at Bernie's 2 was right about the animating abilities of music.
Or they could go in another direction, and just branch out, get serious dramatic actors, comedians, dancers, whatever. Like the big pie fight scene in Blazing Saddles, but everyone involved is a big name actor or actress. And it lasts for two hours. And they use bullets and knives instead of pies. Or in addition to pies. Whichever.
But definitely have Meryl Streep and Morgan Freeman be the bad guys. You telling me Streep can't befuddle Stallone with some cutting remark, then stab him in the eye? She's Meryl Streep, of course she can! She probably even has a couple of shivs she made out of spare Oscars just for kicks, maybe while she was in a crafting phase. Morgan Freeman lulls a couple of the good guys in with his soothing narrator voice, then BAM! duel-wielding machine guns! He said he liked to play bad guys, because they're the most interesting and generally guaranteed to survive until the end of the film, so let's hook both these fine thespians up with the opportunity to go nuts.