my idea of a tech tree would follow a realistic approach. For example: you start with few pieces, crappy engines and so on. So you cant do for example the interplanetary travel from the beginning. to unlock more powerful engines you have to "collect" materials from mun or minmus. So you have to accomplish a human or rover munar mission first, collect a certain amount of material, bring them back ( with even the extra weight considered ) and when you succesfully end the mission the more powerful engines are unlocked and the option "interplanetary travel" is present in the tech tree. At that point you will be allowed to leave the kerbin sphere of influence. At the same time to be able to build and send a rover on the moon you have first to accomplish a mission on kerbin, for example build a rover, drive it around for tot km and than the "rover" is present in the tech tree and from that moment you can build any kind of rover and send it into space. To be able to perform a manned space mission you would need to unlock the "manned mission" in tech tree, and to do that first you have to complete a manned orbit around kerbit and bring back alive your astronaut. If you want to be able to do rendezvouz in space you have to first send two astronauts on a kerbin orbit and train them to rendezvouz, otherwise you wont have the rendezvous ability in the tech tree and wont be able to use it in your space mission. Another example: before you can land anything succesfully on mun you have to send a probe and establish an orbit under a determinate heigh around mun, otherwise all your mun landing would result in a crash, and that will unlock the "land on mun" in tech tree. And so on following this spirit.