It's difficult to regard any 40 something actor in this generation that matches up to De Niro or Pacino for how great and iconic they were. If I was to try and pick then I guess Di Caprio? Maybe Bale? I don't believe these 2 have starred together have they?
It's worth bearing in mind that both De Niro and Pacino were in their 50's when Heat was released, so including actors in their 50's you could add a few more actors into the equation such as Pitt, Depp, McConaughey, or Cruise who is just out of his 50's. I don't think Depp and Pitt, or Depp and Cruise have shared a screen together and those are some of the biggest middle-aged acting names around who haven't starred together. But debatable whether they're up there with De Nrio's and Pacino's acting ability.
I remember the hype in 1995, and there is nothing currently that is quite as compelling as Pacino and DeNiro sharing the screen and facing off in crime drama. I approve of many prior suggestions of top tier A-Listers who haven’t worked together.
The following are my hypothetical contributions that may have been interesting, or still could be:
The mafia/crime route:
Relative newcomers in 1990 in Best Pic nominees, Goodfellas & Godfather III were Ray Liotta (RIP) and Andy Garcia, respectively. Sad it didn’t happen!
The auteur filmmaker route:
Best Pic winner, No Country For Old Men featured the aging Tommy Lee Jones outfoxed by young bucks, James Brolin, Javier Bardum and Woody Harrelson, the characters had limited interactions. It would be interesting to see them butt heads again when they are Jones’ age in another rugged, Coen crime drama.
From space fantasy to the streets: Ewan McGregor & Liam Neeson?
Comedy duos mixing it up in something different: Jim Carrey/Jeff Daniels, or Keanu Reeves/Alex Winter, or Mike Myers/Dana Carvey?
The ultimate = Daniel Craig vs. Tom Cruise in action/spy caper with new original characters?
The thing with Pacino and DeNiro originated from the fact that they co-starred in a classic film once, but due to the flashback narrative, couldn’t act with each other.
This happened again very similarly in Oliver Stone’s JFK where Kevin Costner plays DA Garrison investigating the assassination, and Gary Oldman plays Oswald in flashbacks, who’s character is already dead as the film begins.
Has Costner ever worked with Oldman?
Also, I’d like to see the 90s Batmen together, Keaton, Kilmer & Clooney!! Clooney should play the villain since his version sucked balls. :-D