Route 44, Blackthorn City, and the northern half of Route 45 should've been snowy.

Mahogany Town, Route 43, and the Lake of Rage should've had patches of snow on the ground and in the trees like what they did in Platinum.

It would've given Johto a bit more variety in the terrain, and given them an excuse to fix the Pokémon distribution too. Also, I just think Blackthorn City isn't as impactful as other 8th Gym cities like Sunyshore or Opelucid. They should've put in stone roads at least.