Alright, I'mma post this revision of something I've been thinking of that, IMO, would be one BAMF ROM Hack. First of all, it would be able to be done with HG/SS Platform, because you'd start in Kanto - if my lurking has taught me well enough, you just can edit everything but the locations themselves for 4th Gen games. Next, this would have NO New Pokemon, but instead have a majority of Kanto, an amount of sinnoh and a couple from Johto and Hoenn, none from Unova. Now, I don't have alot of things worked out for details, but I can show quite something for it...
NOTE: Keep in mind I'm NOT ROM Hacking this, but it would be awesome if I knew how. *Sigh*
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...Oh! You've arrived! Welcome to the world of Pokemon!
*Oak Sprite fades in*
*Standard stuff is stated, with selections kinda similar to Pt, and spriting similar to HG/SS, but with either Brown, Black, Grey and heavily Blue/Pink(Gender) or in an alter version with Grey, White, Yellow and Blue/Pink(Gender)*
The different versions are Pokemon: Void Version and Pokemon: Infinite Version
*You wake up in your home, in Pallet Town. After choosing your starter, your rival(Havn't decided) challenges you. Standard stuff up until just after beating Brock; Then it gets a bit wierd.*
Oak's Dialog:
Oh... this is bad. ↓
It appears that Professor Rowen was right. Here, hold on. ↓
*Professor Oak takes your dex in a moment, and tinkers it. It now has a collection of all Kanto Pokemon, several Sinnoh Pokemon and a selection of Johto and Hoenn Pokemon*
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So on a nd so forth, I could go further but I say against it. As you can tell, it has a LOT to do with Sinnoh and Kanto. Why? Because of something undecided aon sending, you guessed it, either Arceus or the Creation Trio to Kanto. In Pokemon: Void, Giratina would be found, as well as Palkia and Dialga - they would be forcing Pokemon to migrate, and eventually you have to stop Giratina, who is controlling the other two. Not sure how it would work. In Pokemon: Infinite, Arceus is found... Doing pretty much the same thing. but I say; I think it's got enough bulk story-wise to use.
BTW, I'm thinking the Rival would be someone from *Kanto, I'm thinking with final team of *F/W/G Kanto Starter*,(Rival-ness) Granbull,(Johto native, they use Snubbull directly after beating Brock, before you can go to the next town.) Altaria,(Hoenn native, see: Granbull) Machamp, Alakazam(/Kanto) and Garchomp.(/Sinnoh)
Now how's that sound as a whole?