An interesting discussion about builders, sorry to barge in in the middle
While I agree in principle that the tasks they do can be integrated into the build queue, having a physical unit on the map does add some extra thrill and avenues. What about all those builders you take from the barbs or enemies? Back then when they were immortal, spotting an unprotected builder did lead to quite a number of early wars, I'm sure, so that was one of the factors that could define the direction your game would take. Even with limited charges as they are now, it is always satisfying to source some builders from outside for free, isn't it? Aztecs can almost run their entire empire on "guest" workers only. How would you integrate this aspect into the build queue model? Build a raiding party project with an rng chance?
Making improvements dependent on the technologies you research was a master stroke for CivRev, but it was developed as a much faster paced game for consoles and fumbling with the controller - the less units you have to move with it the better, hence another neat idea of grouping 3 units into armies, borrowed so efficiently into Civ 6
On the whole, I like builders best as they are now, with limited charges and instant improvements and present on the map. But yes, give them ability to build roads after some tech is researched. The current charge price and limitation to the ME is outrageous, even more so, as after a while you can lay railroads at will, free of any charges, just using some resources.