I just had this idea come to me. Haven't tried it in play, but seems like it would work and be cool.
To be able to unlock space flight, you have to have a minimum of 14 techs (4 of them need to be a level 1, level 2, level 3, and a level 4).
14 techs is the min techs needed in the tree to unlock space flight. But, now, if you wanted too, you could unlock 1 level 1, 1 level 2, 1 level 3, 6 level 4 and be able to play for a while with some of that level 4 tech in play.
Again, I am at work and not at home to look at the techs to see how unbalancing that would be. You could also say they need some minimum number of each before unlocking the next level or maybe you could say you need a minimum number of lower levels to unlock the higher levels (ie, you need a minimum of 2 techs to unlock level 2 - so two level 1 techs, a minimum of 4 techs to unlock level 3 - so two level 2 and 2 level 2 - or 3 level 1 and 1 level 2 , and a min of 6 techs to unlock level 4 - so any combination of the previous).
Something to play around with when I get home.