If the only goal in your Pokemon RP is long-term, activity will inevitably die down. Players need objectives, or at least ideas for objectives; an incentive to post other than "I'll reach that one gym leader eventually". That and it's the GM's responsibility to lead rather than to sit back and let the sandbox run its magic while gradually introducing new routes and areas. That might work for a month or two but things'll grind to a halt when players get busy/demotivated.
Do any "veteran" PC roleplayers know of a journey RP that lasted past the first few gyms? Crossroads got past Bugzy, I believe. That's honestly the best I've seen thus far.
I'd like to run a "journey" RP of mine own, eventually. Something with goals and a plot too it. We'll see.
Yeah, people should look at the Adoption Thread more. We need an audience to make it work. Though you'd probably need to suggest more than just "a journey RP".