Hmm I have a few suggestions.
I found Platinum significantly harder than Diamond when I played through both on a Nuzlocke (I never played Pearl but it's the same as Diamond w/ a few different Pokemon). There's also more stuff to do in Platinum, like move tutors, and you can get a few Pokemon that you would not normally get before the end of the game, e.g. Eevee and Togepi.
I didn't find Emerald that much harder than Ruby until I got to Tate & Liza, because they have 2 Pokemon each, and their Xatu likes to mass spam Calm Mind, then proceed sweep you.
That level curb is not easy to get over, especially when you arrive at Virdian Forest with L50+ Pokemon and all the wild Pokemon are L4 Caterpie. Then again, I guess it's just more tedious than challenging; you just have to spend more time training.