Characters from Vertigo, Image, IDW, Dynamite, etc. Never thought I’d see Dr. Manhattan on the same page interacting with Superman, but we got that. Dream of the Endless has made 2 big cameos in DC stories (and several little ones). Death had a lengthy encounter with Lex Luthor. The Teenage Mutant Ninja Turtles crossover with Batman was so successful it’s basically it’s own universe now. The Power Rangers met the Justice League, etc etc.
So if we can’t see a crossover with Marvel characters, what’s a crossover you’d like to see?