I've probably mentioned this several times before, but what about randomized tech trees? Each tech could have in addition to a base level of each of its prerequisites a maximum positive variance (which determines how much the prerequisite level can vary) and a probability of occurrence. So if Phased Polaron Beams required level 5 physics with a 2-level variance and a 80% probability then they could appear anywhere from level 3 to level 7 of physics, or they could simply not appear one out of five times. Additionally, you could have a game option which sets whether each race gets their own uniquely randomized tech tree (balanced of course in some fashion so one race doesn't get all the fancy techs early and another never gets anything
) or if all races in the game get the same tech tree. Imagine what that could do to tech trading, assuming (unlike MOO3 which implemented a similar system) that each race does not know without using intel what another race's tech tree looks like!
