You sure Dr Doom isnt as smart as Thanos but he nearly is. Thanos has Dr Doom under constant survelliance. Batman has been able to outthink Darkseid at one point. They would be a deadly combination.
__________________ Watch what people are cynical about, and one can often discover what they lack.
- General George Patton Jr
Dr. Doom is incredibly smart. Thanos is far smarter though. Thanos is possibly one of the most cunning of all of the Marvel villains. He has outsmarted Death, Galactus, in-betweener, all of the elders of the universe, Silver Surfer, etc..
And there's a problem of what Doom would do with his prep exactly that would help him beat someone at Thanos' level. When he has been that powerful, it's always through a plot device like a Cosmic Cube or the Beyonder. Since those aren't readily at hand, what would he do?
Intelligence only goes so far when you don't have enough resources to match power.
Batman outsmarted Darkseid once in COSMIC ODYSSEY, by having Dr. Fate keep an eye on him, and intercede at the right moment. On his own, not much he can offer besides guts.
The other time he crossed Darkseid, he was lucky to get away with his life, thanks to the scripting of Jeph Loeb.
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"I'm not smart so much as I am not dumb." - Harlan Ellison
Doom did steal Galactus's power so that sends tons of respect his way from me. With a week of prep,,,,Doom and Batman figure out every angle that Thanos would or could possibly take and then take him down somehow for the win.