Looked around a bit and didn't see this suggestion, so thought I'd throw it out there: adding part reliability to the game (at least as a career mode toggle-able option). As a systems engineer myself and having worked closely during my career with several space launch system and satellite development teams, building reliability into space and launch systems is a first order consideration, and sometimes the overwhelming consideration during the design process. The longer the mission and more complex the satellite/probe, the greater cost premium we're willing to pay for high reliability parts or designed-in redundancy in order to improve our chances of a successful mission. Not only could this make the game more fun by increasing the "pucker factor" on complex missions and launchers, it'd be a great educational experience teaching folks the value of the KISS (keep it simple stupid) principle of spacecraft design. Could also create fun rescue mission scenarios when your Kerbals are stranded in Jool orbit with a dead main engine. This would be a simple addition I think. Assign a failure rate and a couple or few failure modes (from the part just stops working all the way to the part explodes damaging other parts) to every part in the game. Each part has a cost range instead of a single cost - cheap parts only have 1/10 the reliability of the premium parts.
