Question World Generation for old server

So my server is 6 years old. I was always updating the bukkit and now I'm running on 1.12.
When I'm generating new world, it's amazing. Crazy mountains, forests etc. But my good ol' world doesn't seem to get the same treatment. Even though I'm discovering new terrain that wasn't discovered before, it's still plain old. I do get the villages, ocean temples, horses and llamas, but no fancy new biomes. Is there something I should change in server settings? Or maybe it's like a chain reaction with the old generator and I have to look further?
Any advice will be appreciated.

 

@MikeGold
It may just be that the seed does not generate those biomes close to you. Eventually, you should be able to find it. How far have you checked, and in how many directions have you checked?

 

Around 3000 blocks each direction. But in all fairness You might be right and I could have missed them. I don't want to auto generate the terrain to that extent as it will be really hard to get to new content when Mojang will add even more new stuff.