I'm worried tech diffusion has gone too far the other way now (it's too powerful). I played a game last night where I was able to easily exploit tech diffusion for essentially free techs. My strategy was to research a techs the AI did not have, and those techs took me 50-100 turns in the classical era. Then, I would be far enough behind in the other areas of the tech tree that tech diffusion would allow me to grab the rest of the techs in the previous row in 1, 2, or 3 turns. I basically didn't even have to care about investing in research, I could have run a full-culture/espionage economy and gotten free techs from the AI's as they gave me tech diffusion.
I'm not sure I think that's so bad - with conditions.
I understand that Open Borders affects Tech Diffusion (TD), which is good, in my view. If my citizens are able to freely move across a friendly border, then I would expect a strong flow of knowledge, both ways. This seems reasonable to me. Conversely, if I'm isolated and my people believe that we are the only Civilisation in the Universe, TD should be very very low.
You mention that you consider it to be an issue that one is able to run an Empire effectively without investing at all in technology... Fair enough. Is it possible for TD to vary also by the specific technology itself? For example, farming (Agriculture) is easy for a citizen to see and copy and therefore has a high TD rate, whereas say Flintlock (or whatever) isn't and would diffuse only slowly. The idea is that military technologies (say), which your empire would certainly not be trying to share, would diffuse slowly, whereas common civilian technologies would diffuse quickly. This way you'd have to do some research of your own, but weak Civs
couldn't fall too far behind; they'd generally have access to reasonably modern infrastructure, if not military technology. They might occasionally get stuck at a bottleneck technology which has a low TD rate for a while, then after they learn that they'd quickly start to catch up again.
I realise that tech's aren't purely military / civilian, but the rate of TD would be, after all, entirely arbitrary and could be based on any game-play factor we like.
I would envisage that this woud involve giving all technologies a single extra number (int would probably do) and a minimal extra calculation when TD rate is calculated.
Perhaps we'd no longer need "tech' welfare", though I'm not familiar with the details of how that works.
Silly? Over-complicated? Doesn't solve the issue?
I have so many ideas! I should learn to mod' myself (extensive Python experience, and CivII "mod" experience).
Cheers, A.