More bang for your buck - What would you like next for KSP?

[Wahgineer]

Implementation of the Tiberdyne Aerospace Shuttle Booster System, which was very recently updated to use only stock resources and, thanks to the new super-beefed 0.22 A/SAS, no longer requires an off-set COM compensator plugin. Ksp could use some 3m parts, as well an actual use for the very shuttle-esque Mk 3 plane parts. This could also be used as an excuse to upgrade and re-texture the plane parts.

[Highlad]

Definitely not realistic ...

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Also killing a Kerbal (or even a whole capsule crew) as condition for GAME OVER could make the game rather short

Yeah, I know its not actually realistic, I figure i would put it in inverted commas. I was thinking of it as more of a super hard difficulty more than a realistic space program.

[raygundan]

5) Rebalance the tech tree so that it can only be completed by 'mining' all bodies in the system for all science types

That's a bit extreme. The "kerbal universe" should contain more possible science points than it takes to unlock the full (current) tree. Not everyone is going to get everywhere, particularly if they're limited in the parts they have to do it. I can't count the number of attempts I made to land on Eve and return before I succeeded-- if I had been limited to only part of the tree because all the Eve science was required to get the last unlocks, I'm not sure I ever would have made it.

What might work, to go along with some other things you suggested like "better nuke engines further in the tech tree," is putting some seriously ultra-high-tech Kerbal rockets that don't currently exist further on as "completist unlocks." But I don't think making the current tech tree require all science on every body is going to do much besides raise a frustrating barrier for people who aren't ultra-hardcore that drives them away from using career mode at all.

In addition, there's very little point in having a tech tree that is only fully unlocked upon total completion even for the hardcore. What are you going to do with your newly-unlocked whizbang engines if they only appear after you've already been everywhere and done everything? If a part is going to be used, it needs to be in your hands before you've finished the game.