upgrades to existing parts higher in tech tree than the top parts

[thereaverofdarkness]

I've noticed that when I play a game of KSP, I put a lot of effort into unlocking the tech tree. Once I have done that, I feel like there's not so much left to do. I could go explore Duna again but it's more fun when I'm also completing some goal. The problem is that I don't have all of the cool parts unlocked until I reach the top of the tech tree, so I get used to living without them and wishing I had them to complete my goals. I never have the top parts when I still have tech to unlock.

So I was thinking it might be nice both to add upgraded variants of parts, using lighter, stronger, and more expensive materials--but also giving rise to more room in the tech tree beyond unlocking all the parts.

Also I think more of the basic structural components should be available early on. It doesn't make much sense to me that I can build a 2.5 meter liquid fuel engine but I can't build a 2.5 meter empty box to adapt it to a smaller rocket on top. And it really doesn't make sense that I have advanced jet engines, yet I still can't make 1x1 meter metal squares.