Upgrading parts

[Crusher8000]

With upgrading buildings and earning experience for the Kerbals we could also consider making parts up-gradable especially since the tech tree tends to be finished quite fast. Engines could have a mark I, II, III, .... which improves power and ISP but at an increase of cost.

I heard that upgrading parts wasn't considered to be a good idea because it would complicate sharing rockets but if you can keep the previous versions available (you don't always need the most powerful version) it shouldn't complicate things too much. It would lengthen the time you unlock all the best parts by a considerable margin especially if you make the upgrades pretty expensive.

Tanks could contain more fuel and add an ISP bonus.

Engines could increase in power and ISP

RCS tanks get more fuel

RCS thrusters could be more powerful

Solar panels become more efficient

Pods could increase in RCS fuel and power supply

Wheels get more grip

etc.

[Kegereneku]

Myself, I would use upgrade as a gameplay mechanic to balance progression.

Engines would start nerfed and rise in FEW big step until they reach their maximum.

The idea isn't to have upgrade things for the sake of upgrading, it would only concern the most important component.

The goal being to make you switch between differents technology and design all along the game, if you didn't used much solid booster you might reconsider seeing the Mark II is "50% better".

Gameplay speaking it would lengthen the early game fun of -trying- to get into orbit.

New player would have more time to familiarize themselves with the control before being expected to orbit.

While veteran like me would be more challenged trying to orbit as soon as possible, even with underpowered rocket.

Again, it would only concern rocket engines and power source once there's more equipment using it. Needing to upgrade anything for the sake of upgrading would only bore players to death in my opinion.

Logarithmic upgrade is also a sin to avoid at all cost. It might sound realist but it isn't FUN to spend 10hours grinding for a mere 1% upgrade (and worse if you can't know if you are making progress)

I'm still hoping the Devs will rework fundamentally the tech-tree.