After tallying a $174 million opening-weekend haul, most studios would've wasted no time announcing that work already had begun on a sequel. However, Marvel isn't most studios.

Director Shane Black's Iron Man 3 has already grossed about $950 million worldwide, making it the most successful installment of the series and the second-highest grossing film in the Marvel Cinematic Universe (after the $1.5 billion Avengers, naturally). Yet, an enormous question mark dangles over Iron Man 4.

That's partly because of the way the studio has constructed its feature slate, as interconnected films released in phases, with each wave building toward an installment of The Avengers. The Marvel calendar is essentially set in stone through 2015, which means even if a fourth Iron Man is in the studio's Phase Three plans, alongside Ant-Man and Doctor Strange, it wouldn't arrive in theaters before May 2016.

But there's also the matter of Robert Downey Jr., who's made no secret that he's thinking about retiring from the role that's made him fabulously wealthy -- with box-office bonuses and backend compensation, he earned $50 million from The Avengers alone, many times more than any of his co-stars -- and, more importantly, a bankable star. (It's hard to believe that just a decade ago, Mel Gibson had to pay his friend's insurance bond for The Singing Detective, enabling Downey to leave behind stints in jail and rehab and begin a career comeback.)

"It got me thinking about how big the message from your cosmic sponsor needs to be before you pick it up," Downey recently told GQ, referring to an injury he suffered on the set of Iron Man 3. "How many genre movies can I do? How many follow-ups to a successful follow-up are actually fun? Because, as quiet as it's kept, I come from a family of very innovative writers and directors and actors and artists, and the circle of friends they were in were the people I heard having pun-offs playing poker at two in the morning, and it was just the most comforting aspect of my childhood. So there's this kind of legacy of souls from what I consider to be a very particular time in entertainment, and I'm sensing a return to that — it's what me and the missus are doing next. It's not unlike: I heard Brady signed on for three more years with New England, and then he's done being a QB, because he'll be 40. I'm 47, and I'll be 50."

With the release of Iron Man 3, the actor has fulfilled the unprecedented multi-picture deal he signed with Marvel in 2007, leading to the current high-stakes negotiations for his involvement in The Avengers 2, which opens May 1, 2015, and The Avengers 3, which presumably will close out Phase 3 in May 2017. Downey will turn 52 the month before.

If, as Downey contends, Marvel is "so pissed" that the terms of his contract, and the record-breaking performance of The Avengers, resulted in that $50 million payday -- not to mention a reported $35 million already for Iron Man 3 -- what's the likelihood that the notoriously frugal studio and the actor's agents are going to settle on a number that encompasses both Avengers sequels and a fourth Iron Man? If we're to believe the latest reports, Iron Man 4 isn't even part of the current discussions. Where then does that leave Tony Stark, to say nothing of the Marvel Cinematic Universe?

Given that the Avengers sequel will have to move into production by next spring (writer/director Joss Whedon has already completed the first draft), it seems the studio has little choice but to pay Downey whatever it takes for him to reprise his role in that film. Beyond, that, short of a three-picture contract that would undoubtedly push the actor's paycheck into the nine-figure range, Marvel's options are somewhat limited:

"Retire" Iron Man

The Avengers' comic-book history is one of change, with the team's original roster being completely replaced by the 16th issue. It's already been confirmed that Quicksilver and the Scarlet Witch are introduced in Whedon's sequel draft, so it's not difficult to imagine that The Avengers 2 could see the old order changeth, to borrow a phrase, with new heroes entering as familiar ones exit. Why couldn't Tony Stark decide to hang up his helmet by the movie's end?

With properties ranging from Ant-Man to Doctor Strange to the Runaways in development, and Blade, Daredevil, the Punisher and Ghost Rider now back in Marvel's hands, the studio isn't short on potential new franchises. (Remember, it wasn't that long ago that Iron Man was a second-tier character, unknown outside of comic-book circles). If Marvel can repeat the formula that's been so successful for the past five years, combining a rising leading actor, an unlikely director (someone from television, or from outside the blockbuster arena), and a script that blends lots of action with doses of humor -- all on a tight budget -- then it's certainly not impossible to repeat the success of Iron Man.

Of course, the studio's other solo movies have yet to reach the heights of that franchise, a fact that's been attributed directly to Downey.

Recast Iron Man

Retiring the character would mean sacrificing Marvel's golden-armored franchise goose. So what about recasting Tony Stark with an actor who would be happy with a paycheck somewhere in the $1 million to $5 million range? In a word: No.

While other players in the Marvel Cinematic Universe have proved replaceable -- three actors have portrayed Bruce Banner, two have been James Rhodes and, as of Thor: The Dark World, Fandril -- Robert Downey Jr. is Iron Man. "The other individual franchises -- Thor, Captain America, Hulk -- etc., don’t have near the level of box-office potential that Iron Man does,” analyst Doug Cruetz told The Hollywood Reporter. “The other way to look at it is that Iron Man would probably look more like those other franchises in terms of box-office performance without Downey.”

But just as the comics have given us multiple Avengers lineups, they've also presented us with James Rhodes as Iron Man, at least for a time. Still, Don Cheadle is no Robert Downey.

Kill Iron Man

Although this route wouldn't solve any financial problems created by Downey's absence, it would make a tremendous statement that Marvel is willing to do what Warner Bros. so obviously wasn't with its Batman trilogy: kill off its big hero (at least until enough time has passed for a reboot).

Tony Stark was ready to sacrifice himself to save the planet in The Avengers -- that's something Downey, Black and Marvel Studios President Kevin Feige have underscored in interviews about Iron Man 3 -- so his death at the end of the sequel would create the perfect thematic bookend while also generating a ripple effect throughout the subsequent solo films. If Agent Coulson's murder, or "death," is what spurred the heroes into action against Loki and his allies, what would the death of Iron Man do?