There are two separate issues here. Firstly, the implementation of bridges and heights and their relation with one another is the same in Extended as in Standard, so what you request in the first paragraph applies equally to the Standard code. It would be a huge undertaking for me to make that sort of change, as I do not know much about the inner workings of the code, so, if I were to do it, it is likely to be very many years before this has a higher priority than other tasks.
As to the second paragraph, this is a very complex issue. Firstly, there is not a consistent scale relating to height (and it is difficult to create a consistent scale because of the restriction in the different possible degrees of height). This means, in turn, that Secondly, there is a very straightforward way of building a double height bridge, by holding down CTRL when building a bridge. There is no equivalent for bridges of greater height. Thirdly, it would be extraordinarily difficult to communicate in a clear and consistent way to the player precisely what height of bridge is required for which vehicle. Finally, there are already difficulties with towns building low bridges over rivers, which I plan to address when I have time. It is very unclear how such a solution could work if there were an unbounded range of different heights required for different vehicles. In reality, of course, towns often build bridges over rivers too low for tall sailing ships to pass under them - but not too low for small sailing barges to pass under them (a Thames sailing barge, but not a tea clipper, can pass under London Bridge, for instance).