After playing KSP for a while, I noticed something. After playing career mode for a while, Science mode can feel too easy. After all, you start with all building upgrades, so you can gather much more science faster, and build bigger vehicles, right at the start. Without the limitations placed on you by building upgrades, Science feels like there's not enough progression.
I propose a mode that is similar to Science, where there aren't funds, contracts, strategies, or reputation to manage. However, the buildings would start at Level 1, and would be upgraded through Science. They could either be unlocked in the tech tree at certain tiers, or be purchased like they are in Career, with early-game science yields perhaps boosted slightly to account for building-upgrade purchasing.
I think that this would be a good mode to have, as it places some restrictions on Science mode while not having any other stuff to manage such as in Career mode.