You have the Galaxy Trio on the bottom left, with Frankenstein Jr. behind them. On the car (a revamped Speed Buggy?) is what I'm guessing is an updated version of The Impossibles (the springy guy, Coil Man, gives it away...I think the female character is new, since they were a trio as well). Up front is Johnny, Hadji and Bandit from Johnny Quest. Above them, Space Ghost, Jan, Jayce and Blip (who is a monkey from space, but NOT a Space Monkey), with Biiiiiiiiiiirdman and Avenger next to them. They're surrounded by the various members of The Herculoids, the only names of which I remember are the amoeba-looking Gleep and Gloop.
That just leaves the guy with the horns over Johnny's shoulder. I genuinely don't remember him.
If I was a time traveling kid from the '80s, this would be the coolest looking thing I'd ever seen, as long as they promised to kill off the Herculoids in the first issue, because the Herculoids sucked. Except for the dragon that shot lazers out of its eyes. (Zot? Zok? I wanna go with Zok.)