>>1465 (OP)
If you're looking for advice I have one for your design process.
Realistically what you do is, rather than take the time and effort to learn and make people learn an entirely new system, your primary design document is going to be the 'vibe' of the campaign, and then you bastardize D&D into it, by basically stripping away anything that doesn't fit the vibe. This is how a LOT of old JRPGs were codified.
This means design your major narrative structure FIRST.Then write down a few concept encounters that are effectively setpieces you'd like to do. Then design your primary antagonist, this genre is very much carried by how impressive its villains are. Flesh out the villain's personality and motivation, and like maybe one or two powers. Then make some interesting NPCs, try to make a vertical slice of who you want players to meet to the theme of the JRPG 'feel', a giant monster for an encounter, an evil lieutenant, a mid-level friendly NPC, a comic relief NPC that gets shit on a lot, a really, REALLY weird guy(a lot of games and shows have at least one), and maybe one nice villager NPC. Stop IMMEDIATELY when you feel the least bit bored making characters, do NOT push yourself to keep making more, only work with cliches and archetypes that interest you at this point. Then define the PRIMARY locations they will be going to, go from MOST to LEAST interesting, preferably ending with your starting town. All of this is to establish your main framework of how this campaign feels, the 'vibe'.
Then set your level parameters for the campaign so you limit the over the top teleport behind you powers. Next think about the most powerful abilities you want to have show up, as these are always showstoppers in these things. This is where you bastardize mechanics to fit the setting, just rips pieces out here and there from something generic like D&D, whatever everyone is most familiar with. Do NOT let the faggot urges take over and pick some obscure game nobody plays as your host system. You want something that feels familiar so that the players can relax a little and not powergame the system and descend into a boring grind UNLESS they think it would be funny to do so, which is basically the only time I would allow for it, since I prefer milestone levelling. Only mine splatbooks if you're completely out of ideas for how to make something work, and even then don't obsess.
Then once you've come up with a few parameters here and there, refine your story, add characters and items and places ONLY as needed to facilitate the story that you structured earlier.
Do NOT, under any circumstances, do large portions of worldbuilding and mechanics until the very very VERY end, preferably not at fucking all. THE biggest trap for being a dungeon master is working out all the mechanics, a complete bestiary, 100% of the skills and powers, the entire world map, etc first. Likewise doing too much worldbuilding will bog your setting's sense of mystery down, and discourage players from being interested in it. You have to walk a very fine line in fantasy of not overexplaining your world so that it sets off the brain itch in your players. If you build the entire world FIRST, you will run a colossal risk of generically-downgrading your campaign so that none of all that hard work you did building the setting gets cut. This is an ego trap, you will HAVE to make cuts to finish the project. Don't spend time trying to flesh out an entire world, it isn't worth it, there are video games that already do that and a lot of them suck.
Coming up with what you think is a cool campaign concept and then piecing together whatever makes it work with lollipop sticks and tape isn't glamorous but it produces a better experience overall.