This is an amazing project! Platinum is one of my favourite Pokémon games, it's nice to see someone finally hacking it in style!
Are you going to incorporate Pokémon changes as well? If Drayano is ok with it, you could port his SGSS changes over to Platinum, although it's probably better that you add your own personal touch and think of some other ideas for upgrading Pokémon stats, typing, etc.
I don't really want to change pokemon other than their evolutions. I dont want to change the tms and hms they can learn, their typing, or their level up movesets, because it will cause problems for anyone with flash carts wishing to trade and battle with people who have the actual game.
However, I think I have decided that instead of two difficulties, this game will have one difficulty. I will make the game harder. I also plan on changing all post-game areas so that the pokemon and trainers found there are going to continue level-wise from what levels the elite four had. Although I'll make them slightly lower.
I will use an increased level scale. I have also decided that the first Elite four battles will be against what the game's default second battle is(Cynthias garchomp will be level 78), and their second battle will be all level 100's and will be unlocked after catching Arceus, who requires you to have caught every other pokemon to catch.
I will also take out various trainers, while still making sure the player can see every Sinnoh Pokemon if they battle every trainer, and use their trainer battles to create battles against various champions, elite four members, etc. from previous games. I will also put in battles against teams made by people who have helped me with this game. These will also be all level 100 pokemon and require catching Arceus to fight, but you will be able to fight them an infinite amount of times. They will also be extremely challenging, as I will give them the highest AI that I can, highest IVs, and competitive movesets.
Those are my goals, and those are what I am currently working on.
I think I am also going to put in Barry from the Future who uses Palmer's battle sprite and says he realizes that when he was younger he wasn't the best Pokemon trainer, but through the years he became much better and used celebi to go back in time so that he could teach himself how to be a better pokemon trainer earlier on. I will probably also make his team level 100 and he will require the capture of Arceus to fight.