Yes, Djokovic could be a "great." For a simple reason, he's beaten guys who are talked about as GOAT of Fed and a GOAT of clay. Yes, fed's window has been winding down, but he was pretty much challenging him and winning matches during fed's prime and Nadal is pretty much in his age prime, minus his physical ailments. But he's beaten nadal before nadal had all the knee issues. So anyone who can take down these top two tier GOAT's deserve to be a GOAT himself.
I have immense respect for Djokovic, but more so how he handled Nadal last year at Aussie open and beating him at Indian Wells and Sony Erricson as well as those clay court events. Djokovic is the real deal. He is more mentally strong than people give him. If he is down break point or even match point, he plays that same point as if he was up 40-0 and I would have to say with that courage you are mentally strong as any elite athlete can be.