Tech trees are a staple of the RTS genre. In every game, there must be a feeling of progression - from weaker units, more primitive defenses and smaller production, to stronger units, more advanced defenses and larger production. Part of this progression involves researching technology and having access to different unit types, after certain pre-requisites have been met. You can't begin producing advanced ground until you've built an advanced ground facility. You can't build super weapons right out of the gate, etc.
Although I have ideas for what techs, I've saved this particular game mechanic for last. You sort of need to have the full picture to get an idea of what technologies would improve gameplay and create that feeling of progression. Thus, I am waiting until all of the units are completed before inventing techs and have nothing to show on that front.
I can say, however, that it is a major goal to have technologies that don't infulence gameplay in an artifial way. To me that means the effects of techs should be apparent within the game's graphics. If aircraft can move faster as a result of a tech, perhaps they should have a different color burner. If cannons gain an increased blast radius, their explosions should be larger. Simply changing unit stats without giving and indication to the player is a lazy way to implement tech upgrades.