I thought it would be nice to have actual mission planning in game for things like complex maneuvers, orbital slingshots, Lagrange point orbits, and the like.

Then launch countdowns, maps, exact timed burns that you'd still have to do manually but you could seemingly do so much more.

Or maybe this kind of thing already exists with mods.

Mechjeb does a lot of automation for complex maneuvers like planetary transfers, ascent autopilot, target interception, etc.. You can even do a porkchop plot for interplanetary transfer. but lagrange points are not possible in KSP because of the Sphere of Influence Model. and orbital slingshots are not included, I assume because they require an intense bruteforce calculation.