Let's be careful here. I've heard this complaint so many times, that God has to do some great miraculous thing to prove He exists before someone will believe. The problem with this is man's stubbornness to accept something he doesn't want to accept. Case in point, the Holocaust happened what, 70 years ago? There are vast amounts of photographic, audio, and eyewitness evidence that it happened. The camps are still there and can be visited. It is perhaps the most documented atrocity in human history, yet there are many who today deny it ever happened.
Here's the truth. God could come down to the 50-yard line in the Super Bowl, proclaim Himself Lord of all, do all sorts of miraculous things, and what would be the response? "Hey, neat special effects", and within a decade or so, vast numbers of people would refuse to believe it ever happened. I call it, "Do a trick" theology, and God would have to show up over and over again, doing tricks to convince the stubborn that He exists. The "evidence" that you're looking for is something you discount or ignore, which is the changed lives and testimonies of those who not only encounter Jesus but put their faith in Him. You readily cast aside the testimonies of millions throughout the years and ignore them.
Be careful what you insist must happen before you believe. You might just get it.
When I am alone or working in the yard or not doing anything else, I listen to debates between Christians and atheists. Matt Dillahunty is one of those I listen to frequently although I have heard every debate between Dawkins, Hitchens, Sam Harris, and scores of others, with some of them being former pastors turned atheists. Matt was asked what it would take for him to become a believer again. He was a bible scholar and Baptist before and used to preach to others.
Your question is hypothetical and also vague. When someone says "God" it is often a sleight of hand because they switch back and forth between a generic "God" and a Christian god concept when convenient. So, what God are you referring to? The Christian one? Allah? Thor, Ra, Mithra, Zuess, or some god that came and went centuries ago?
You say Jesus has changed the lives of many people and of course, I have heard this claim a million times over. Why is Jesus not seen or heard when people make the claim that "Jesus turned my life around"? No one ever sees him when Tom a lifelong addict says he found Jesus and quit. When you delve into it you find that it is his BELIEF that Jesus turned his life around that did it. You could insert Allah, Brahman, Norse Paganism, etc.
So, what god would it be on that Super Bowl field if "God" appeared? Not one reader here would say it was any god but their own. How could you tell? The pictures of Jesus in Medieval times make him look like an unattractive dark-skinned Middle Easterner. Today he looks like a handsome movie star.
What would it take? I dunno. What if he started by saying he cured childhood cancer today and people in the stands called their friends who were desperate for their cancer-ridden child to be cured and thousands of people in the stands murmur that children with cancer immediately got up and danced.
But no. The imaginary god does none of that. The world operates as if there is no god and all is the luck or bad luck of the draw. Tens of thousands killed in earthquakes, tsunamis, floods, disease, war, and criminals. When Joe Bloke CLAIMS that Jesus made him stop doing X we readily believe it and yet millions of kids are tortured or raped by maniacs every day or die of disease or cancer and you god sits idly by.
We are what we believe we are and if John quits drugs, extramarital sex, or smoking it is his BELIEF that some strawman god named Brahman or Alalh did it.