RE: Pokemon Amethyst Version
I haven't seen a game on this forum for a while... But so far, you've just got ideas. I guess it's a bit early, but if you can get some screenshots or examples of maps up, it would make the thread look better (as well as giving people a reason to contribute).
Anyway, I'll try to give you some tips on what you've got so far. I see you're basing the story on Cresselia. However, using Cresselia's power to create a new universe is a bit of a stretch; Cresselia's power is nowhere near that of Dialga's or Palkia's, and besides, it's the legendary of dreams.
Second, what are Team Galactic doing in your new region? They were based in Sinnoh, and broke up after the events of D/P/Pt... I guess that's similar to G/S/C, but Team Rocket were just generic bad guys, while Galactic was focused on using Dialga/Palkia's power (hence the name). I would make a brand new evil team, and change their goal to fit Cresselia's power.
A similar thing applies for Cresselia itself. It's native to Sinnoh; what's it doing in your region? You could use this as a story element; perhaps start the story in Sinnoh, and have something happen to Cresselia causing the player to follow it to the new region. It would be interesting if traveling between the two regions was worked into the plot; perhaps after a certain amount of time in the new region, an event occurs that sends you back to Sinnoh?
About sprites. Are you going to use the D/P/Pt sprites for everything, other than the custom trainers? How about tiles? The original D/P/Pt tiles don't look so good in 2D (if you're using RMXP, you won't be able to make a 3D game)... There are plenty of tilesets around; if you're going to include Sinnoh, I suggest you find Kyledove's custom D/P tileset. As a warning, that tileset is one of the most popular; unless you make some custom tiles too, people may whine at you for being 'unoriginal'.
Charizard88 is in charge of making the actual game in RPG Maker, right? Has he ever used it before, and if so, is he experienced with it? Trust me, you don't want a sub-par game. Also, since you have multiple mappers, each one will require a copy of RPG Maker, as well as be able to work together in terms of map style and contents. After all, you don't want a horribly put-together region, do you?
Finally, you'll want to learn scripting. Although there aren't many other games here, if you decide to expand to another forum, then you'll want the game to stand out. Unless you get a decent scripter, this is quite tough. With a scripter, you can create new features for your game, such as custom menus and minigames. Those are generally the things which make a game different from others; if your game hasn't got any custom features, most people won't be very interested.
EDIT: Why shouldn't Darkrai be in the game? Are you seriously already planning a future game already? The reason Dialga/Palkia aren't in the opposite versions is because Diamond is a counterpart to Pearl, not a sequel. Note how both of them play a role in Platinum. If counterparts are what you mean, what's the point? Nintendo only makes two versions to make more money; some people will buy both to get all the Pokémon. You're not trying to make money, so you may as well save time by just making one version. At least you could have Darkrai appear in the story, but be unobtainable (like Dialga/Palkia in Platinum, before the National Dex). That way, when you do make a sequel, it can tie in directly with events in this game.
As a side note, the name 'Pokémon Amethyst' appears to be popular >_>