So I've been playing through Diamond again recently (in a big Sinnoh mood right now, that region is special to me), and there's a bit of a peeve that I have with DP's level curve, especially around the mid-game point.
The game based on the Trainer ID case and many official sources suggest that after Gardenia, you should do Maylene third and Crasher Wake fourth before battling Fantina. Even the anime follows this progression. The way they supposedly want you to travel based on that is to go to Solaceon after Hearthome, then go up Route 210, avoid the foggy half, walk straight down to Route 215 into Veilstone, then battle Maylene, then down Route 214 and 213 and into Pastoria, battle Wake, then walk down 212 and back into Hearthome. Sounds like a pretty natural progression....right?
Except...the level curve of the routes completely contradicts this. Route 212's Trainers have lower leveled Pokemon than the Pokemon in Routes 213-215 and even the foggy half of 210 and 211. It's actually closer to the point you will be in by the time you reach Solaceon. Furthermore, the 212 Hearthome gate is locked only until you've visited Solaceon, which means you can go there right away once you've passed Route 209 and visited there. Furthermore, the Gym Trainers in Wake's Gym are actually lower leveled than those in Maylene's Gym.
Which basically means...the level curve and the requirement to unlock Route 212 suggests that the game contradictorily wants you to actually battle Wake first, then go do Maylene after you've defeated Wake. The level curve suggests that they want you to visit Solaceon, do part of Route 210, then go back to Hearthome, travel down Route 212, visit Pastoria and battle Wake, then go up Routes 213 and 214 and then visit Veilstone, and battle Maylene next. Then head up Routes 215 and the northern half of 210 with the foggy stuff and into Celestic Town and the eastern half of Route 211, then go back to Hearthome.
And while that's...not necessarily a bad order, it requires quite a bit of backtracking. But that's not what annoys me here. What does annoy me about this is how incredibly contradictory it is. The so-called "official" order of the Gyms from there is Maylene->Crasher Wake->Fantina, but the level curve wants you to do it Crasher Wake->Maylene->Fantina. It's like two different people decided the order of progression they wanted you to challenge Maylene and Wake but one person named it based on a linear route progression and another decided the level curve very differently and wanted players to do it a different way.
It's a good thing Platinum rectified it by creating a smoother level curve towards its intended order (Fantina->Maylene->Wake) and also locking you from doing Wake beforehand, and the game's level curve aligning with that as well as the route access restrictions making it clear what order they want you to do it in.
DP's level curve also suggests that they want you to use the Vs. Seeker and battle several Trainers whose levels go up with each rematch in each of the old routes, considering how incredibly steep it is. While it does incentivize revisiting places you've already been to, and I don't really mind doing it, many players probably cannot be bothered to go through all the effort to find all the Trainers who have upgraded rematch levels and battle them for the exp, especially having to revisit several routes multiple times. And they want you to do this multiple times throughout the main story. First right after you obtain it, they want you to go back, then again after Fantina, then again after say, Byron and Candice, and whatnot. It's quite difficult to regularly keep track of all these Trainers, and it's worse when whether they respond to the Vs. Seeker or not is entirely random and if they don't respond, you have to walk another 100 steps to recharge then try again. It also seemingly wants you to do the Water Routes (219-211) immediately after getting Surf.
I love the Sinnoh games still, but this is a bit of a peeve that I have with it in terms of the level curve. Now granted, it's no where near as bad as Johto's level curve, but the main thing is that it makes it so you can battle Maylene and Wake in any order, but the official order and the level curve order contradict each other so heavily that I just really find it baffling, and wonder who designed the "official order" and the level curve.