I just thought it could be fun to have real "choices" in the techs selection. Let's imagine you have techs A and B, with A unlocking A1 and A2, B unlocking B1 and B2. If you pick A, you can never pick B anymore (hence B1 oe B2). Could be a way to further specialize your civ depending on the resources (mines, farms or materials) it has around for the green tree, specialize into some kind of strategies/equipments in the red one (swords over maces for example), into a branch of spellbooks, etc. There could then be a C tech (not necessarily in the SAME tree) which, depending on wether you chose A or B, leads to a D or E techs. 

An example : imagine you have a mine. You have choice in the green tree between improving food, material, or metal production. You pick metal production. It would allow you to pick more exclusive techs (but never to pick food or material again) with buildings like, for instance, smith (+25% metal prod). It would ALSO unlock some "metal weapons and armors" techs in the red tree. You obviously have NO NEED for those if you have no metal (there are other resources like metal that keep you away from using equipments, like cristal or horses, which could be involved in the choices). You would have real choice to make in green tech tree, which would influence the red one. It would prevent the situation in which the one with the highest research score (for instance one with a library right next to his capital) gets all techs and owns everyone. Well, chosing metal over food wouldn't mean you have NO FOOD building at all, but you have BETTER or MORE metal related ones. Same with military : chosing swords over maces you could still have some rudimentary maces, but would have exclusive excellent swords.

There could be a lot of work on spellbooks and spells through that system too. It would be more intersting to have you choose your spellbooks that way and then have a rather specialized spellcaster relying on your ingame choices, than to just pick all spellbooks in character creation. If you pick earth, then no air. Fire, no ice. You could chose your elements depending on the available shards (who never picked half the spellbooks and ended up with the shards from the exact other half ??) You could still choose to have enchantment , summoning or combat spellbooks at character creation (the "arcane" or general ones). Then with the magical rings tech, you would have fire OR ice magical rings, but never both.