One of the many scenarios I play in my head alot is the concept of a fused universe between Marvel and DC. I try, to the best of my comic book knowledge which isnt all that formidable, look at it from a controlled, historical standpoint opposed to the wildly imaginative "Anything goes" view that probably most fans who would bother to consider such trivial matters would probably take where I see almost all of the truely MAJOR fictional events still taking place along with many of the more minor details as well while also taking real world events into perspective in a unrealistic-realistic balance.
I see the Timely/Atlas/Marvel universe sort of fitting into the folds of DC Universe in alot of ways only because DC seemed to break alot of ground early such as inventing what is widely considered the first full-fledged superheroes with Superman and Batman as well as the first superteams of both the golden and silver ages with the JSA and JLA respectively. Ofcourse, once the "Marvel Age of Comics" took place Marvel gained atleast equal footing in the industry as far as innovation and fandom goes.
A few random thoughts:
-The original Human Torch and the Submariner would have probably been charter members of the JSA. Captain America would likiely be a shoe-in as well once he made his debut, perhaps as the appointed leader during the peroid where the JSA placed itself under the auspices of the United States War Department as the Justice Battalion? A few other Timely heroes would likely also gain JSA membership such as the Angel, Vision, and Miss America who actually had pretty long print runs in the Golden Age along with other characters who garnered enhanced status retroactively during the late 60's to mid 90's.
Other Golden Age and retroactive Golden Age groups such as the Seven Soldies, All-Winner Squad, Invaders, Liberty Legion, and the All-Star Squadron would still have taken place with varied line-ups.
-I envision the JLA still forming with maybe some of the Timely/Atlas/early Marvel characters also as charter members, followed by the Fantastic Four who probably have many early team-ups with the Justice League, and the Avengers also coming into fruition as a second superhero collective in such a massive universe would seem likely.
-I see Spider-Man and Johnny Stomy being atleast honorary, if not charter members, of the Teen Titans. I think they would be viewed with respect by many of the Titans as the first major solo teen hero and first "equal" partner in a adult super-team respectively. The Titans would also have countless crossovers with fellow teen group the X-Men and would count many former members the original X-Men, New Mutants, Generation X, and other young mutants along the various Titan line-ups as well.
-Speaking of the X-Men, many DC characters were originally described as mutants before the advent of the "blanket term" metahuman used to describe basically every super-powered character in the DCU such as Captain Comet, Brainwave, Mammoth and Shimmer, ect. I also see other fully human character with "inherited" powers such as Jade, Obsidian, Atom-Smasher, Fire, Airwave III, Brainwave Jr., ect as being considered mutants and perhaps taking part in various mutant groups.