I would find it much easier to believe in God or 'a God' if there was something concrete I could put my hands on. Such as rocks and water and the like.
It would also help if that matter was so utterly incomprehensible, that science wouldn't be able to so easily agree on it's base components and origin.
If life were more of a mystery and not so easily understood it would be helpful. As it is now it all clearly explains itself.
If the universe was infinite, 'or so appeared', and otherwise incomprehendible, it would make it easier to believe in 'the other'.
If even without a cohesive natural explanation there was a mysterious need for a moral framework in man, that would make it easier.
If God would come on a space ship or even better as one of us, and clearly speak the truth and show the way.
If while here, He could perform miracles, and then die for me to prove who He is, that would be best case evidence.
And If He promised to make right, every foul thing in life, while somehow not taking away my free will, I would be impressed.
And last but not least, If He gave me a manual or something to decipher all of this mess, I would be obliged.
Other than that, I don't know what else He could do without totally imposing His perfect reality on people who really don't want it if it doesn't include their sin of choice.
Any biters in the stream?