WE are talking about Augstine of Hippo, right? Not St Augustine the Apostle of the Saxons?
I'm fairly sure he would have made his mark anyway, being a gifted rthetiorician and philosopher. However, he could not have become the head of either a pagan or a Christian seat of learning as he was a Manichee before he converted. Thus, an unconverted Augustine would continue to teach Gnostic truths as he saw them. He might get in trouble with the Christian authorities (unless he finds a powerful protector) and find relatively little support from the pagan side (as he can accommodate pagan deities in his belief system, but not regard them highly, they being of this world). Wherther we would know anything of him will largely deepend onm whetherthe church develops in a similar manner without him, or whether the absence of 'de civitate dei' actually butterflies into something big and painful. Maybe the whole 'blueprint for a Christian state' that this boook is often read as is replaced by millennial expectation which, in turn, is bound to be disappointed and create heresies and spinoffs? A disintegrating Christianity would have a decent chance of preserving the memory of the great Manichaean teacher as the founder of some heretical movement or other?
Of course, if you want to go all-out weird, you could have him turn into an inspired Manichaean missionary who converts all of Africa to the Gospel of Sophia...