Physical Technology prerequisites
For whatever reason, this came to me today as I was thinking about Elemental. Classic technology trees are pretty much entirely under the players control as far as being able to research them (barring random techs). You research Tech A and you can get to Tech B (or C and D, etc). Occasionally it'll have something like "build a mage tower" before you can research it.
What I'd like to see is some techs or maybe entire trees/subtrees depend on accomplishing something in the game world. You can't research Necromancy techs until you've found the ArchLich and defeated him in battle (a hero quest). Or maybe you can't research a tech until you've struck a peace agreement with the "Keepers of the Fallen Library", a small cult that survived the world's destruction and has a library of ancient techs.
You get the idea. This would force turtlers to get out in the world a bit. It might also give you a potential strategic point where you really want to hold the territory around the "Keepers of the Fallen Library" to stop other players from accomplishing the task that lets them dig into that tech tree.
It certainly wouldn't be something for every tech or tree, but it'd be kind of cool to take some of the tech research control out of the players immediate control and make them go out in the world and hunt for the knowledge.