If they were to do another set of Iron Man films, I'd start with the Believe Armor
And then do something like so.
Iron Man 4
Introduce Count Nefaria, playing the Dr. Doom level role of the MCU. He's mastered nano-tech which starts to uncannily resemble asgardian "magic" and he's pretty much incredible as a physical and intellectual threat. He goes head to head with Tony, who (recast) is trying to rediscover himself. Shades of Secret Origin of Tony Stark (but without the brother) and Stark Disassembled, where Tony has to get rebooted. In finding himself and his origin, Tony is reborn for a new generation, and with a new actor. This time jaunt folds well with the idea of Doomquest, or in this case Nefariaquest, where Tony and Count Nefaria are going head to head, seeing as how they are both utterly awesome technologists who feel as though they can affect the past. They may not go to Camelot this time, but perhaps a period piece in the 20s could be fun. This is where we'd introduce his daughter Whitney Cabe/Madame Masque as a love interest and then villain. Good times.
Count Nefaria: Someone awesome like George Clooney
Madame Masque: Someone hot, and deadly, like Malin Ackerman could be great.
Iron Man 5
Ezekiel Stane has basically hacked himself and made him some biological armor. That could be totally awesome technology, plus he has a crazy vendetta against Tony for, y'know, killing his dad, and bumping him and his trust fund out of the company. I think it'd be incredibly cool to employ the Ghost as that kind of corporate saboteur as a secondary villain. Madame Masque would also still play as a villain/love interest.
Having him employ/deploy popular Iron Man villains as hacked bio-armor would be great. That way you could also have Ghost, Living Laser, Grey Gargoyle, Melter and Blizzard. You could take those classic characters and do the stuff from the Five Nightmares and the Believe arc, where he has a race against the clock to fix this huge problem. Totally high octane fast paced. Tony's arc here would be a lot simpler, but mostly it would revolve around trying to deal with hacking himself like Ezekiel... how human does he have to be to fix the problems?
Zeke Stane: There are many options, I think Justin Timberlake could do a great job. Yeah I said it. The others would be great in the hands of a number of great character actors. I wouldn't give arcs to those other characters, they'd be like Coldblood and Firebrand in that way, simplified, distilled into incredible physical challenges with fun packages.
Iron Man 6
I have no idea. Ultimo? That being an ancient powered armor that Tony discovers and realizes some things are beyond his control. Tony being sure he's going to die and having to deal with his mortality, his legacy? That could be cool. Let Madame Masque and maybe the Hood come full circle as his enemies, but Ultimo be a problem he has unearthed, against his better judgment that stands to possibly destroy the world.
So, overall, Tony has to deal with who he is, what he is and then what he leaves behind. Sounds like a cool set of arcs.