There is a lot of fun that could be done to spice up the techtree a bit. Here are some stuff from the top of my head. 

One shot techs.

There could be a few techs that would only be available to the first one who research them. When an empire has researched it, everyone gets a message about it and its gone from the tech tree. Anyone currently researching this tech would loose the investment (should not be very expensive techs).

Examples could be:

Hunters cabin (building). Gives all bows extra damage against beast. 

Statue of the wanderer (building). Gives +3 prestige. When first built will attract a high level hero to your cause. This hero also brings a decent army of mid level units with him

Annullment (spell - can be cast ONCE). Will remove half the gold, mana, iron and crystal from all empires except the castes.

Seduction (spell - can be cast ONCE). Will take 5% of the population from all other players and add to your own.