For a fantasy mod I'm working on I'd like each civilization to be able to choose one or two of a small set of key technologies that will lead to the bulk of powerful tech. Tentatively these are Magic, Religion, Mechanics and Nature (and possibly Forbidden Knowledge, but I haven't decided on that yet). I want it to be possible to either focus completely on one branch or to split between two of them (combining Magic and Mechanics, for example), with unique technologies being available depending on which one(s) you choose. However, it should not be possible to go into more than two of these groups, and if you go for both Magic and Mechanics you shouldn't get access to all the technologies of the "pure" trees of either type. That's the goal, but I don't know how to get there. I could possibly do it with events that give breakthroughs for certain technologies - for example, all the techs in the Magic branch are hideously expensive until you research the first (cheap) one, and if you do that an event triggers that give you breakthroughs on all the others, effectively reducing their cost to a manageable level, but the event won't trigger if you have already researched two of the other "base" techs. However, this seems very clunky and risky - if something goes wrong with the event triggers, you'll be completely screwed. It would be possible to do it with leaders, but I want to keep those two choices distinct. I definitely don't want to key it off which civilization you pick - part of the fun is having the elves being just as able to go full out Mechanics as the dwarves. Is there some option I've overlooked?