Well, Thanos has "succeeded" in taking over the whole Marvel Universe three times. He destroyed half of all living beings in the Universe with the Inifinity Gauntlet once too.
I think it was Warlock who convinced Thanos that he didn't truly want to rule the Universe because each time he attained his goal, he sabotaged himself and let the universe get away again.
I think he's one of the most dangerous villains, ever. Certainly the most dangerous in Marvel.
Marvel, Black Adam, Power Girl, and Sentinel together have the raw power to takedown an unprepared Thanos. Thanos could match any two of them, but not all four at once.
Now that's FOUR magically powered powerhouses anchoring the JSA. There's a chance that Warlock could turn that tide; e.g. if he can easily drop a few of them ("disrupt their mana-centers"😉 with karmic-blasts, or something.
Flash is the other wildcard, and the Hawks are useless.
I'll say two to one odds for JSA