So let's talk about the gamma-irradiated elephant in the room, the fan "demand" for an MCU Planet Hulk film that pops up every now and then. Evidently, there was a "backlash" that portions of the Planet Hulk narrative and imagery found their way into Thor Ragnarok, rather than being its own film. Now, I wasn't regularly reading new comics when the Planet Hulk story-line was running, so I'm not sure how "big" it was, but it is certainly one of the classic Hulk stories and a really fun read. That said, I'm really not sure how well it would translate into a film. There's a reason the Iron Man film franchise didn't do direct adaptations of "Demon in a Bottle", "Extremis" and "Five Nightmares", but rather amalgamated the parts of the stories that worked in the MCU milieu to create interesting cinema. I really don't think that the MCU Hulk, as much fun as he is, could carry a film that would primarily be CGI and motion-capture, like Avatar, and have it be to the same level of Marvel Studios efforts like... pick anything from Civil War onward, really.
If you have to be a completionist, check out Planet Hulk, but I really don't recommend it over the comics or Thor Ragnarok, especially as both are now really easy to get your hands on. I can't even say that this works for children, as the amount of blood and fighting ups the rating to M 15+ here in Australia, so, again, you may as well slap Thor Ragnarok in the DVD player or cue it up on demand. Also, because I know someone's thinking it, Taika Waititi is the best Korg, don't even at me, nerds. Maybe one day, when the MCU has rebooted, we'll get a "proper" Planet Hulk film, but don't hold your breath for Mark Ruffalo to be signing up for it anytime soon. Also, Red King is just kind of a crummy villain, I think people forget that because the rest of the characters work so well. Food for thought.