I finally had the time to reread H.P. Lovecraft's THE WHISPERER IN DARKNESS, and though the majority of HPL stories probably register only as "null-crossovers," since the icons referenced don't appear "on stage," WHISPERER actually qualifies as a full icon-crossover, for reasons set down here.
An example of a time-travel crossover set in modern times:
And just to give an example of a "team" of innominate legends drawn purely from recorded history, here's a 1947 BLUE BEETLE story from ALL TOP COMICS #8. The titular hero encounters, thanks to a time-travel device, a "super-menace team-up" whose members are culled from different eras: the pirate Blackbeard, the serial killer Jack the Ripper, and the wife-murderer Doctor Crippen. In fact, the writer of this tale made an overt attempt to "mythify" the historical Crippen-- who only killed one woman according to the law-- into some sort of odd "Bluebeard" type who killed multiple wives.