I would like to see Dr. Doom have to contend with the Juggernaut. I'd prefer a more classic Doom than the one currently cosplaying as Iron Man, but take what you could get, I suppose.
It doesn't seem hard to the two together. Doom's knowledge of magic could lead him to try and take the Ruby of Cytorrak to use as a power source. Which could in turn get him in hot water with Cytorrak himself, leading to some sort of team-up between Doom, the Juggernaut, and Dr. Strange (who doesn't want Earth getting destroyed because Doom went and pissed off an extra-dimensional being).
Or I could see Doom trying to either hire or manipulate Juggernaut into attacking someplace. Probably not to acquire something Doom wants, because I doubt he'd trust the Juggernaut not to break it. Plus, even if the authorities couldn't stop Juggernaut, they could follow him pretty easily, and Doom probably wants more plausible deniability. If Doom tries to double-cross Juggy, or blames him for screwing something up, there's the conflict.
The other option was for Juggernaut to simply come barging into Latveria because it happens to be along the most direct line to get to wherever he wants to go. How does Doom handle that? Direct confrontation? A trap? Does he try to divert Juggernaut, convince him to take a different route? Would he try to protect his subjects, or would he decide the risk of being humiliated was too great and simply stay home, monitoring the situation? There's a decent argument to be made the best option with Cain Marko is to keep his path clear and not get him riled up. Don't give him an excuse to start smashing through things, and, assuming he's most concerned with getting where he's going, he probably won't start anything to delay himself. On the other hand, if Doom lets this guy just barge into his country, stomp through it as he pleases, and walk right out the other side, that could make him look pretty bad. Not just to the Latverians, but possibly the larger international community. "Doom can't protect his borders," they might whisper.
Although given how often the Fantastic Four come barging in, they might be saying that anyway. But Doom pretty much always makes the attempt to stop the accursed Richards and his allies. Not even trying to might convey a different message.
You could still do a variation on this in his current status, with Doom coming across the Juggernaut (whoever that is at the moment) causing trouble somewhere and stepping in to try and put a halt to it. Is he focusing on protecting innocents in the line of fire, or in confronting the threat? What kind of plan can he come up with to win the day (if he can)? Magic, or science, or money? In the old days, I could see Doom possibly trying to quietly buy Cain Marko off if he was losing the fight (I can't see him resorting to that until after he's started fighting him, even though offering him some money and a ride to the next country would be the quickest, quietest solution).
Heck, maybe Doom can pull it off. If anyone has the combination of will, smarts, and resources to stop the unstoppable, it's Dr. Doom.