[Suicide Junkie]

It could also explain the racial techs (e.g. crystal techies can't grasp that organic tech because some of the theories they rely on say it's impossible, and vice-versa; those who buy both traits have a different set of theories that work and aren't mutually exclusive). Also, I was focusing more on the philisophical side.

I consider the Organic and Crystalline techs to be somewhat borderline.
Psychic, Temporal, & Religious are more easily seen to be something inherent to your race's biology, at least by me.

The kind of thing you'd need thousands to millions to hundreds of millions of years to change, and a re well outside the scope of the game.

Although, again, planet and star creation should also take more than thousands of years to settle down to something usable.

Yes ... but remember, A was better than C in every way possible; assuming they have the same research costs, it then becomes a matter of choosing short-term or long-term. Likewise, I didn't address what comes later down the line in tech A....

Fair enough. There are decent ways to weave it into gameplay.

Again, an abstract mechanic could represent this by having say, plate armor and cavalry become unavailable, or at least much harder to produce, after gunpowder weapons reach an advanced state. It's not really an accurate cause and effect, however. The real cause is a shift in infrastructure and trained workforce (technology) and socio-economic effects, neither of which are really included in SE4.

I'm really starting to like the idea of giving greater and greater cost reductions for each item made using one repeat-build order...
Something like 1% cheaper after every 5000kt of resource spent, up to a limit of 20% cheaper.

* One way to achieve this in modded SE4 is to have major theoretical prerequisites which make it possible but impractical (or at least, a major trade-off) to try to research alternate branches. It would also help if the research points, costs, and tree depths were sufficient to avoid players "getting to the end and having nothing better to do with their research points".

A good, practical idea.
First you need to make sure the alternatives are all viable, though (and in different ways). None of the APB/PPB we see in unmodded SE4; more like SE3.