The only thing left IMHO is The Ten Rings. Fulfill what they've set up over three movies already (4 if you count Ant-Man). Give us straight up Fin Fang Foom, Mandarin and all that jazz. One last ride, Tony. Come on!
There does feel like some good comics mythology to pull from as well as stuff still to mine that they've already set up in the films, whether it's characters they've already introduced or set up or threads like the Ten Rings/Mandarin or AIM. In that sense, an Iron Man 4 almost seems like it should be made just due to the missed potential in IM2 and 3 (and I actually liked both films) because even if they carried along Tony's character arc well enough, they missed an opportunity to flesh out "his world" better and this may be the only time we get Iron Man stories any time soon.
The only question is where can you go with Tony's character from here? Not even talking comic storylines but just in the MCU. I can't help but think Infinity War will feel like the climax not just of the Avengers saga, but the end of a journey for a lot of the characters. Maybe topping that or adding on more baggage isn't the solution, maybe making it way more personal is the way to go with the theme of Tony sowing the seeds of his own personal demons growing in the shadows. That's always been pretty compelling for Tony so far, but they've turned the threats he's helped create more external after IM3 and problems that effect everyone. Tying that into the legacy of his father, for better or worse, is also something he can't escape. Maybe Tony just trying to finally step away from being a superhero and just focusing on being an inventor and a business man again is where he'll find himself, which actually would be a good way to carry on from the ending of IM3, using his inventions to try to better the world or something and the implications that has.
What if after Infinity War, Tony decides to help with Earth's dealing with space threats, whether it's in a positive way by exploring and researching the universe and all these new cosmic alien races they just found out about (perfect way to tie in a Fantastic Four intro if that is a deal that ends up happening) or in a more violent way like the government wanting to build bigger weapons and defense systems. In that sense, it could be a way to explore Iron Man and FF's more Cold War roots without having to deal directly with the actual Cold War implications in a modern world. There could essentially be a "space race" and and "arms race" but rather than the idea of it being to compete with the Russians or Communists, it could be the idea of the quick advancement of technology in the modern age and exploring new concepts that we are still struggling with due to things like globalization and the internet. The world can be best trying to figure out how to deal with the bigger cosmic universe out there after realizing just how big it is; do you attempt to make friends with all these beings like the Asgardians or do you stockpile more weapons? Is it a political game of building alliances and using existing conflicts to the Earth's own ends? Maybe there are already worlds out there thinking of using the Earth as a chess piece for their own agenda. There is certainly some potential for allegory and metaphor there and not only that, it can tie into something big again with the cosmic side of things since there is a big GotG trilogy conclusion I'd assume that's still on the horizon and The Inhumans as well.
Hah, I was talking about them making it more personal but then went off on a tangent about space. Hey, I guess that could be the way they tie things to Fing Fang Foom as well as the Mandarin. Mandarin could be competing against Tony's efforts to establish the methods Earth deals with other worlds (with Mandarin's business efforts and negotiations being nefarious and self-serving for personal gain) but at first they think he's a legit business mogul before realizing his ties to the Ten Rings and his shady ways he made his fortune, as well as his real motivations for doing what he is doing, trying to find the source of his ten rings power knowing they came from space or something.
I don't know, there is probably a story there that would be better than the outline I could imagine. I'd definitely be down for more Iron Man though.