Okay, this has been bothering me a lot so I just want to make it absolutely clear what my issues with this are.
The first and most glaring issue with this project is expecting it to work as a ROM hack. Pokémon Black 2's ROM size is 256Mb, and the NDS ROM limit is 512Mb. (Someone correct me if my exact numbers are wrong here). Expecting all the data for 15 entire regions to fit within those parameters is incredibly questionable, and speaks to your inexperience as a ROM hacker and programmer. How do you expect to be able to work around these limitations? You briefly mentioned that a possible solution would be to split the game up between several different ROM hacks. That brings up another issue, though.
Assuming that the RAM works similarly to the GBA, you're similarly limited in that manor. You can't have an infinitely big save data file. Keeping this in mind, how would you plan to keep all of the flags and variables that track progress contained within a limited area like that, with 15 regions worth of information? Even if you're splitting up the game between different ROMs, how do you expect to make the progress transferable? It's possible to make a tool that transfers relevant Pokémon data, but do you even know how to go about doing something like that? There's just too much going on with 15 regions that make it impractical as one continuous adventure.
Another issue I have with the concept of having so many regions in one hack is making sure that gameplay remains interesting throughout the course of the game. Traditional Pokémon titles already become relatively stale to continue playing around level 50 when most Pokémon stop evolving and learning new moves. What could you do to make the game more interesting, enough so to make a 15-region game playable? Increasing the level cap, while certainly a possibility, doesn't solve this. There's simply not much more you can do within the Pokémon battling system to keep progress interesting for much longer. I think it's certainly doable to get closer to level 100, but after 2.5 regions in Glazed my whole team was close to level 100, high 90s at least. 13 more regions after that? You'd need a major overhaul. You could certainly space out the currently available moves to make progression constant throughout 250 or so levels, but that would make everything much more tedious and progress much less fast-paced and rewarding. Another issue with increasing level cap like that would be that it'd be incredibly tedious to bring a weak Pokémon up to the standards of your team with a system like this around the late-game. Tedious gameplay is not good for making a fun game.
Yet another issue I have with the concept of having 15 regions in a single Pokémon game is that it's a super unoriginal concept. You've listed places that you plan on being available to explore, and none of them are brand-new places. Most hardcore Pokémon players (The vast majority of people playing ROM hacks), are already going to be fairly familiar with most of these regions. Even if they're not familiar with them through traditional games, there have certainly been popular ROM hacks to also visit these locations. At best they'll be shallow rehashes of the same things we've seen in other games as well, just with less ability to have a sense of progress due to the level scaling issues.
Time investment is also a huge issue I have with this game concept. There's two ends of this though: The player's end, and the developer's end. If you manage to put out a hack that's frankly not a super new idea, that's generally gonna be fine. A lot of people will still play it just because they're looking for new stuff to play. But when you expand that to get to 15 regions of gameplay, that's a LOT of time to be putting in as a player. Especially without 802 Pokémon to fill it up, the gameplay is going to be boring, and most players probably won't sit through that much gameplay. The other side of the time investment issue is on your end, as the developer. The best ROM hacks from the most talented hackers usually take 1.5-2 years just to get one top-quality region completed and interesting. By that rate, you're looking at a 2030 release date minimum to complete 15 regions of similar quality. In a 20-year development window, you're not gonna skate by super easy. There will definitely be a lot of setbacks and things that make a project as big as this hard.
That's a problem, though, because you've clearly shown that you're unable to deal with even the most minor of setbacks for projects that you work on. I'll cite your previous post on Pokécommunity for this, you wrote out this big project that you wanted to start as a community to build an entire Pokémon game from the ground up, with its own world, working from nothing. You were looking for help, ready to be in it for the long haul to get it done. Or, really, you just said you were. Because after all of that, your post got locked because you posted it to the wrong subforum. Sure, a simple mistake, one that everyone could make, but that's not the point. A person who is truly determined enough to follow through with their ideas and complete projects would have immediately reposted it into the proper forum, but from the looks of it you immediately abandoned the idea at the first little hiccup before anything even got started. You're clearly unable to deal with the struggles of game development.
All of these issues come down to your inexperience with ROM hacking. You've already expressed here that you're completely unfamiliar with the tools you're aiming to accomplish all of this with. Why post what you're going to be doing before you even know what is possible? When I was 9 I really wanted to make a Pokémon game that had 20 regions in it. Then I grew up and learned more about programming and ROM hacking and game design, and realized that quantity isn't the mark of a good game. I learned I'd rather have a fun game than a game with 20 regions.
I don't think you understand the scale of the project you're proposing. I already briefly mentioned how long it would realistically take based on what we can see from the development of popular ROM hacks, but there's more to it than that. You really just don't have the experience to make all of these promises. You posted in here that you said you estimated it'd take around 3 years before you have even a hint of an Alpha. Two main issues with this. Firstly, that doesn't make sense, because if you're doing 1 region at a time, you could easily get one of them out by that point. My other issue ist that when I inquired about how you came to that figure, you didn't have a solid answer or reference to any real thing besides just saying something along the lines of "I know how long these things take", even though you clearly don't have the experience to know. It's completely reasonable for everyone in here and all the people talking about this in the Discord to assume that you don't have the work ethic to follow through on this. Not only because you already have a track record of not finishing what you start, but also because it's a frankly inhuman amount of work ethic necessary for a project this big. 20 years of work for a fan game that will never be lucrative is simply not a level of work ethic we've seen as a ROM hacking community, for any game on any system ever. The closest to that we've seen is from projects that have been repeatedly dropped and later picked up by different individuals to reach their end results.
I have a huge issue with your inability to accept criticism, which you've shown in this thread. Just because someone is critical of something you're doing or saying doesn't mean that it's an attack on you, or saying that you're a bad person. It's just people being clear and trying to inform you about the things you're showing you're not informed about. It's not coming from a place of malice. Whining about people coming across as negative doesn't get anything accomplished. Exactly zero games of any genre ever that have gotten remotely popular have come with 100% positivity. If your'e expecting to be the first one that gets through development and release with nobody even hinting at a negative opinion, you're gonna be in for a huge surprise
With all of that in mind, I'd recommend you reevaluate your approach and goals in making your game. Firstly, your goal shouldn't be a content goal like "15 regions of gameplay", but rather a quality goal like "A fun game to play". Secondly, you shouldn't come to specifics about what you want to incorporate into your game before you're adequately familiar with the actual process of ROM hacking. See what's possible first, then work within that to make your game. Finally, learn to accept that not everyone's gonna be 100% positive about everything. Not everything is gonna work out perfectly, every part of ROM hacking can be very tough. That's the world, nothing anyone can do about that. You're not incapable of making a good game, but try to learn as much as you can before you commit yourself to an impossible task.