I would really enjoy seeing Doctor Doom in the DC Universe
Doctor Doom is as formidable as they come in the Marvel universe. He has a legitimate shot at taking over the DC universe if he went there.
Victor Von Doom, aka Doctor Doom, is probably the greatest villain in the Marvel Universe. There are few villains who are as interesting as he is- he's not just some evil cliche, trying to take over the world because... reasons, but a multi-faceted character who genuinely believes that he knows what's best for the world and humanity.
Besides being one of the smartest people on the face of the planet, Doom is also a formidable sorcerer and his armor has allowed him to defeat some of the most powerful beings in all of creation, like the Beyonder or the Silver Surfer. While he hasn't been able to conquer the Marvel Universe (well, keep it conquered), how would Doctor Doom against the heroes of the DC Universe? Would he be able to defeat them and rule the world or would they stop him?
It's debatable whether Doom's magical prowess would cross over between universes, but even if it didn't completely, Doom's base of magical knowledge would serve him well in learning even more magic. The magic of the DC Universe is quite different than that of the Marvel Universe- it's more powerful and one of the fundamental forces of creation.
Doom would work to tap into this new source of power and make himself even more dangerous than he was before, which is bad news for all involved. Well, all involved not named Doom.
Doom's armor doesn't get nearly enough credit for just how formidable it is. It's allowed him to go head to head against some of the most powerful superheroes, villains, and cosmic beings in the Marvel Universe and triumph.
On top of all that, Doom's armor is equipped with a device that can drain power from opponents. He's used it against the likes of Silver Surfer, the Beyonder, and even a rogue Watcher to drain their powers and use them. This makes it pretty much entirely Superman proof, as he can just drain away his energy and become ever more powerful.
Doom has done something most villains haven't- he's won. Doom was able to do what none of the heroes were able to do- not only did he save the universe from destruction but he was also able to become a god and recreate what was left in his own image.
Villains winning is very rare in general and few of them have been able to get wins as big as Doom was able to. A lot of it was dependent on Doom's prior knowledge of his universe and taking advantage of that but Doom is a fast learner- given enough time, he could figure out the ins and outs of the DC Universe and use this knowledge against his new foes.
The thing that makes Doctor Doom so dangerous is that he's not just one threat- he's just not an evil super-genius or an armored titan or a powerful sorcerer. He's all of those things and that's just scratching the surface of his threat.
Doom isn't an easy threat to deal with because he keeps changing it up on his opponents. Doom is impossible to predict and he's smarter than just about any of his adversaries. Confronting Doom is also a difficult thing because he's not going to make it easy on anyone who comes up against him. Doom believes wholeheartedly in his own superiority and is always ready to prove it.
I would also enjoy seeing Batman in the Marvel Universe as BK Ray outlined. So many cool encounters that would occur.
Doom has always been my favorite villain.
I read in some editorial once about how you have to give the villains wins in order to give them credibility and show them to be real threats to the heroes. They have done that with Doom.