So there's going to be meaningful differences from the original game, aside from all the Pokemon changes.
Trying to go through Mt. Coronet to get to Hearthome? Have a Dawn battle. Acting as a proper second rival here, Dawn has a solid team. A Piloswine (underleveled), Clefable, Lopunny (now Normal/Fighting with Scrappy) and Prinplup. She actually took me down the first time, but I was better prepared the second time.
Route 208 is pretty normal, aside from the house that's now selling rare berries. These are the stat-boosting "pinch berries" - Leichi, Salac, etc. However, before you enter Hearthome, you meet Elite Four Aaron. He heals you up and then battles you with a weaker version of his team! Levels 31-32 Dustox, Beautifly, Venomoth, Scizor, Drapion. Even if you lose, you don't black out, but continue on as he gives you TM81: X-Scissor.
Now, the gym is the same in Hearthome. Obviously, Fantina is a lot stronger, with highlights including Calm Mind + Baton Pass Drifblim (which it never used), Gengar and Mismagius packing Dazzling Gleam for Dark types, and 4/6 of her team having Will-o-Wisp. I had to save Zircon for Gengar so I could have a chance to take it out, thanks to Thunder Wave. Anyway, you normally get Shadow Claw from her, but that TM was placed outside the Old Chateau. Instead, Fantina gives you TM30: Shadow Ball!
After you battle your rival Barry again, a maid runs in asking for help at the Pokemon Mansion. While you can step into the grass on Route 209, you're otherwise forced to go the other way around. Along Route 212 and in the mansion, you'll fight Team Galactic again. After each room a Clefairy runs out, except for a Happiny from one of the rooms. They all use Metronome which turns into Teleport to get a Galactic grunt out of the way, and then Happiny gives you the Lucky Egg. I love all the creative little story events this hack adds...
Anyway, you then fight Saturn and Mr. Backlot, the owner of the mansion whom Saturn forced into a double battle. Barry teams up with you, but he hasn't trained a bit since you battled, so no Staraptor or evolved starter yet. They've got a tough team of Bronzong, Octillery, Toxicroak and Wigglytuff, Espeon, Raichu. Possibly the worst part is Raichu's Fake Out, which makes it hard to survive and attack them. It was through this battle that I learned Pokemon still evolve even if they faint in the same battle.
The next Gym is Maylene, which gives us a cap of Level 39. I switched out Labrador (Labradorite was too long) and Zircon for Topaz the Sandshrew and Albite the Staraptor. I've always liked using Probopass in vanilla Platinum, but he's even better here as a Fairy counter. Also, I'm definitely going to need Labrador for Cynthia.