Captain America fits in well with Dc, when it comes to attitude.
The Joker would make a damn fine Spider-man villain.
I’d read that!
Galactus might work as a recurrent cosmic-level menace in the DC universe. Particularly, I could see him and the Guardians in an eons-long “cold war” struggle.
Doom Patrol first appeared June 1963. X-Men first appeared September 1963.
While it is possible, maybe even likely, that different creators shared a drink and discussed some ideas*, given the lead time necessary (especially in 1963) to get an issue out, these ideas likely came up separately.
*Corporate espionage is also a possibility (a leak at the printer, maybe?), but that’s moving into conspiracy theory area.