I don't know about sequel or spinoff but I do have a successor in mind. An anime adaptation of Pokemon Adventures featuring Red as the main protagonist. Honestly I'd think it'd be a really fun adaptation if done right, there's a lot of freedom to expand on in the original fast-paced arc and there's a lot of interesting explanations for much of the story, Pokemon and characters motives and personality based on the games. For example, Red is a battle genius who later starts to get bored of battling later on after he won the Pokemon League because he may have gotten too strong, Green lives under the shadow of his more famous grandpa, Prof. Oak, and has his fair share of run-ins with Oak's old colleagues when Oak was younger, Blue (or what you people know as Leaf) story is probably the darkest under that sly fox-like smile as she was kidnapped by a giant bird and forgotten by the residents of Pallet Town. If you think about it, it's sort of a metaphor how Leaf seems to be the most forgotten female character even though she was designed first before Krystal made her official debut. There's also a very good reason why trainers can't get into Saffron City, because it's simply under Team Rocket's control as opposed to the fact that the guards will let you in if you offer them a drink in the games. And of course, one huge problem the past anime adaptations has is their failure to adapt villains as full-fledge threats. In Pokemon Adventures, they tie up loose ends with a showdown between Red and Giovanni and man, Giovanni does not show any mercy and even teaches Red a few things about what it means to be a trainer. When I say tie up loose ends I mean it, such as how Pokemon Adventures explains the importance of the GS Ball near the endgame of GSC arc whereas the anime kind of ditches the entire storyline.