You guys are really trying your hardest to ignore that I've said it's a GOAL, the execution of which will adapt to what is possible. I'm really not understanding why so many people are opting to be pompous and rude about this.
Ignore this hater I support you 100% I personally believe that when the project is finished that should be the only proof you need as my favorite Latin phrase goes "acta non verba" or "action not words" he is zero help therefore ignore any comment he makes unless he is actually being helpful which clearly he isn't keep it up I hope to one day play this hack myself I don't have much experience in this kinda thing I'm more programmer using Kali Linux than game making but if there is anything you might need program wise I'm more than happy to find it and get it to you just say the word
I think in this situation, it's ok to reach for big goals, and I have read that you would be rehearsed in each program for the accomplishment of this feat, but it's simply too big of a feat in general. It took me nearly two years to NEARLY complete a DEMO for a ROM Hack that went up to Route 4 of Kanto by myself, with subtle references, all-round dialogue changes, custom shinies, custom trainer battles, and all new maps. It was a very difficult feat for a beginner like myself. If your team were to be this versed in hacking to my extent, and have to work on 15 regions, whilst keeping the package interesting for the whole experience, it would be extremely complicated to actually complete to a degree of quality. You would need to make 15 regions worth of interesting (emphasis on interesting), dialogue, battles, stories, references, and maps. As many people have said, the game will get boring. Not to mention, what happens if something is corrupted? In possibly multiple games, with a lack of backups to the GBA ROM, or the NDS ROM? You would need to redo everything that was lost all over again, adding to the time working on the game. This isn't a feat that requires adaptation to issues, it's a feat that requires termination of those issues, and gaining success over them. And, like your project, will take time. It will take time for ROMs to be able to withhold 15 regions, and enough technical prowess to store possibly thousands of scripts.
Furthermore, it's fine if you want to work on this project, no matter how impossible it may seem to be, but when making a hack of this caliber, it's important to put your hacking knowledge into another project of a smaller scale. Work on that first, and see if you can garner an audience, find out your flaws, and what you're good at. A single hour alpha for a niche ROM Hack is a good exercise.
As another note, put this into perspective. Even Game Freak, the creators of Pokemon, struggled to complete Pokemon Gold and Silver due to a lack of file space. Only by the help of Satoru Iwata, could they add only one extra region, a heavily condensed Kanto region. If well-versed game developers struggled at their time to incorporate two entire regions into one game, I'm not sure how a person with unobservable ROM Hacking skills would be able to create a game with not 2, but 15 regions in it. Yes, it was on the Game Boy Colour, in the late 1990s, but the moral is, that if game developers struggled to create their game, there's a high chance the same will happen to you. It will be felt, without access to many tools, and many people on the job to complete this game.
I would like to reiterate as my final message, that it's good to have a passion for projects such as these, but I'm not sure if this dream will be achieved anytime soon, if at all. If you're really determined, instead of making this as your first project, start (and complete to a certain amount) a project on something else, and then move onto this project. This will give you the greatest feel for what it's like to make a game. If you can deal with, the struggles (losing data from corruptions as an example), balancing Pokemon, script making, mapping, story/character-building, and making the game enjoyable for your audience, which is done to quality (as you expect of your hack) for an entire region of your audience's pleasure, then go for it.