Hopefully I can answer some questions, but I'm like half-awake and exhausted from work after spending the entire day trying to catch up on post-Christmas department management/updates, so I imagine my train of thought here isn't as fluid as it should be for answering questions.
I've envisioned this mainly as an open sandbox-like game; most locales I don't have anything strong in mind. If someone wants to say, I don't know, Twinleaf Town had been swept underwater and is now part of the huge, nearby lake or something, all they have to do is run it by me. I'm likely open to most concepts.
On the topic of plot, as with the prior roleplay it happens to be based on, there isn't any overarching plot save for the details listed above. The prior concept was simply "treasure divers in a post-Kyogre world", with the plot essentially boiling down to nothing more than "bad guys appear and cause problems", being the reason that the characters get forced out into the open world. What I envisioned was more akin to probably what Zeroth mentioned, though I can imagine something formulating depending on what sheets people end up making.
Regarding tone, I want to aim for a semi-serious tone, if any of that makes sense. The examples given could essentially work; I just don't want to lean hard into Mad Max Pokémon: the RP or the like. I don't want it to be full on goofy just because it's Pokémon.