He could have been #1 easily at some point during the late 90s and early 2000s. The weak transition period time in mens tennis when Sampras was on decline, Agassi was in and out as usual, and generation next was on the horizon other than the very beatable Hewitt in the early 2000s. However I dont think he will ever get to #1 now as we are already entered into and will continue in a very strong time for mens tennis. Roger will be around several more points and I think the closest Djokovic will ever come to him in the rankings during that time was last year. Nadal will never be surpassed by Djokovic in the rankings until he really begins to slow with age (could happen at 25 or so but that is still over a couple years away). Murray seems to be more a late bloomer type than Djokovic, it is possible now that he is already ranked over Djokovic that he always will be in the future. That isnt even counting Del Potro and many other potential big future champions.
Djokovic could have been #1 in 1998-2002 and probably would have won a bunch of slam titles during this weak period. The period he is coming up in though, no, he is not good enough to ever reach #1 amongst the current field and what he will face in coming years.