James Gunn SHOULD direct, but since this is unlikely to be reversed then the best choice would be Joe and Anthony Russo.
The Russos have already proven they can handle these characters perfectly in Infinity War. They absolutely NAILED the Guardians in both character and tone.
The expected 2020 release is irrelevant. It has not been officially announced, and delivering the best possible final chapter of the Guardians trilogy is much more important than just hitting some arbitrary releases date (which was never official in the first place). And Marvel has already reshuffled a planned release schedule when situations changed.
Waititi could be good, but Im not convinced hes right for the Guardians. His style feels very different. But I KNOW the Russos would be right because theyve already shown that they are.
The question would be would they want to do it? Do they have the energy to jump on this once finished with Avengers 4 next spring? But they are basically a four person team (Markus, McFeely, Joe and Anthony) so they may be able to handle it easier than a single director.