I actually don't know at what point the Silver Age creations were being made specifically to play off the others: rather than just a great variety of titles/ attempts to catch readers with something novel. I realize I'd never pinpointed when the shared world might've emerged from that 'at first' condition. Sort of like looking back into the creative Big Bang of Marvel. Certainly by 'Daredevil' in 1964, the way a character fits into the bigger Marvel picture's becoming part of the thought. (from Captain Marvel #29, 1973, pg. 8 ) Thanos and the company of Titan are generated to play with the rest of the MU. The Avengers title (cover date, Sept. 1963- so, summer of '63) is made to fit the line-up together. Characters are being made for stories, to play off title characters, always. Later characters like Deathlok and Killraven in the 1970s, and Dreadstar in the 1980s, are created as 'stand-alone' storylines- so creativity did eventually