I can't say I like the 1899 tech research model - the year the US patent office was shutdown because "“Everything that can be invented has been” - Once you get to the end of research trees all you can do is trade research for cash, which becomes pointless at some point as well.

I think a mod where you can apply research points to improving the efficiency of all the building in the given tree, say 1% each time with costs increasing in a geometric progression thus providing diminishing returns for your R&D could make endgame research meaningful.