The strong implication seems to be that unless a Jedi is given at least a modicum of training, their force powers never really assert themselves other than as "jedi-like reflexes" and prophetic dreams that they won't really understand. Note that Luke was in his late teens before he even met Obi-Wan and although he was considered to be among the finest pilots of his generation, he'd never been able to do so much as lift a pebble with his Jedi powers.
Yoda is emphatic that Qui-gon shouldn't begin training Anakin until after a decision has been made on his future, the strong implication being that once that line has been crossed, it would be far more dangerous to leave him in-situ.
Had they returned Anakin to Tatooine (or sent him to Naboo) he would likely have led a remarkably mundane life as a hotshot test pilot or a fabulous engineer. There's no special reason to believe that he would have been able to control his powers on his own, or be much of a threat to those around him.